DataStore concurrency test that exposes concurrency issues.

For instance, it exposes the following concurrency issues:
- "refs/heads/master is not a symbolic ref: '<hash>'" IllegalArgumentException at
  FileRefDatabase.getSymRef() when multiple threads create commits.
- NoSuchElementException: No tree ref matched the name: <feature type name> at
  GeoGitDataStore.findTypeRef() on reader thread when multiple threads are reading and writing.
- FileNotFoundException "<path to repo>/.geogit/conflicts" at JEStagingDatabase.getConflicts()
  when multiple threads are reading and writing.
Make sure FileRefDatabase makes consistent reads and writes on ref files.
 Make JEStagingDatabase.*Conflict() methods always synchronize full conflict's file path

This simplifies the code and avoid possible race conditions, and also
doesn't create unnecessary empty conflict files every time ConflictInterceptor
checks whether there are conflicts.
Merge pull request #678 from groldan/concurrency

Fix concurrency issues with the file ref database and JE staging area