DfsBlockCacheConfig: support configurations for dfs cache tables per extensions

DfsBlockCacheConfig: support configurations for dfs cache tables per extensions

Parse configurations for tables containing a set of extensions,
defined in [core "dfs.*"] sections.

Parse configurations for cache tables according to configurations
defined in [core "dfs.*"] git config sections for sets of
extensions. The current [core "dfs"] is the default to any
extension not listed in any other table.

Configuration falls back to the defaults defined in the
DfsBlockCacheConfig.java file when not set on each cache
table configuration.

Sample format for individual cache tables:
In this example:
1. PACK types would go to the "default" table
2. INDEX and BITMAP_INDEX types would go to the
   "multipleExtensionCache" table
3. REFTABLE types would go to the "reftableCache" table

[core "dfs"] // Configuration for the "default" cache table.
  blockSize = 512
  blockLimit = 100
  concurrencyLevel = 5
  (...)

[core "dfs.multipleExtensionCache"]
  packExtensions = "INDEX BITMAP_INDEX"
  blockSize = 512
  blockLimit = 100
  concurrencyLevel = 5
  (...)

[core "dfs.reftableCache"]
  packExtensions = "REFTABLE"
  blockSize = 512
  blockLimit = 100
  concurrencyLevel = 5
  (...)

Change-Id: I0e534e6d78b684832e3d3d269cee2590aa0f1911