Return a new instance from openSystemConfig and openUserConfig

Return a new instance from openSystemConfig and openUserConfig

Move the handling of cached user and system config to getSystemConfig
and getUserConfig methods and revert the implementation of
openSystemConfig and openUserConfig to the old stateless
implementation.

This ensures the open methods respect the passed-in parent config, which
may be different on each invocation. Additionally, returning a new
instance matches the behavior of the previous implementation of the
default system reader, which downstream callers may be depending on.

Move the implementation of the new caching methods getSystemConfig and
getUserConfig up to SystemReader. This avoids that we break the ABI for
subclasses of SystemReader.

Also see [1] which fixed a similar problem with Gerrit's custom
SystemReader.

[1] https://gerrit-review.googlesource.com/c/gerrit/+/225458

Change-Id: If54a2491932d8fc914d4649cb73c9e837c5b8ad0
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>