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 sure JEStagingDatabase makes consistent reads and writes on conflicts files