Use mmapfs storage for LocalClientProvider instead of memory to avoid memory errors

Hello, we were getting OutOfMemoryErrors when using a local elasticsearch instance. The same issue is described in this link:

http://elasticsearch-users.115913.n3.nabble.com/OutOfMemoryError-Direct-buffer-memory-td4032061.html

Unfortunately, we couldn't fix the problem by increasing the JVM's direct buffer memory maximum. In the above link they suggested not using the in-memory index as it bypasses the JVM memory management and is experimental.

This uses mmapfs instead, which should work just as well, and happens to fix our problem.