Clean up `LdapAuthProvider` to properly cover different LDAP search scenarios.

Clean up `LdapAuthProvider` to properly cover different LDAP search scenarios.

Gitblit allows in its configuration to set a "manager" user (and password) which can be used
to search for the entry of a user wanting to log in. If they are both not set, an anonymous search
is attempted. In the description below, when I say "...as manager", it is either as manager or
anonymous.
So far the behaviour of Gitblit, with respect to binding to and searching in LDAP,
has been the following when a user logs in:

**bind as manager**
**search for the user**
_bind as the user_
_search for the teams_

I'll call this code flow A.

Later an additional configuration option had been added: `realm.ldap.bindpattern`.
(PR gitblit/gitblit#162) It was meant to allow for not using a manager nor anonymous binds,
by searching the directory as the user logging in.
This is done in code flow B:

**bind as manager**
_bind as user_
_search for user_
_search for teams_

Both A and B are flawed, I think. In A, it looks like a mistake to me that the binding stays with the
user after authentication. The problem that this causes is, that in LDAP server configurations
where normal users are not allowed to read groups, the team information cannot be retrieved.
I tried but failed to understand how B is supposed to work. There will always be a bind request
as either anonymous or the manager DN when the LDAP connection is created. If neither is
possible, the authentication process will fail and the user cannot log in.

When synchronizing users and teams from LDAP, the following code flow is exercised:

F:
**bind as manager**
**search for users**
**search for teams**

This patch fixes both code flows by introducing a new flow.

C:
**bind as manager**
**search for user**
_bind as user to authenticate_
**bind as manager**
**search for teams**

And it changes code flow B to the following code flow D:

_bind as user_
_search for user_
_search for teams_

With code flows A, C, D and F the following usage (and authentication) scenarios are covered.
They are described from the view of a Gitblit administrator's intent and his LDAP setup.

* Users and team should be snychronized with LDAP
	This means anonymous or a fixed account must be able to read users and groups.
	=> covered by C and F

As the above allows for authentication and is required for synchronisation, all the others below
do not cover synchronization.

* No anonymous binding allowed and no special manager binding required
	This means that users must be able to read user an group entries.
	=> covered by D

* The user DN needs to be searched, e.g. because they are not all under the same parent DN.
	This means that anonymous or a fixed account must be able to read users.
		-- anonymous or the "manager" account can also read groups
			=> covered by C
		-- anonymous or the "manager" account cannot read groups but a user can
			=> covered by A

I therefore believe that the new code will cover all common use cases. The implementation
either directly binds as the user, when `bindpattern` is not empty, or it binds anonymous or
against the manger DN to search for the user DN entry.

If it directly bound against the user DN, the user is already authenticated. It will then only check
that the user DN it found in the search is identical to the one it is currently bound against. If it
was bound against a manager DN (or anonymously) it will bind against the found user DN to
authenticate the user logging in, and will then rebind against the manager DN.

When searching for groups in LDAP, if the search fails with a result code other than SUCCESS,
the implementation will bind against the user DN, if it isn't already bound against it. It will then
repeat the search for groups under the user authorization. This is to keep backwards
compatible with the original behaviour A, in order to not break cases where the LDAP setup
would deny a manager account to search for groups but allow it for normal users.

To achieve this the implementation introduces an internal `LdapConnection` class that wraps
the connection and keeps bind state, so that a rebind as a user is possible.
This also fixes a resource leak where the connection was not closed in case that the initial bind
as the manager account did not succeed.

This commit would fix gitblit/gitblit#920