rResolves #163 replaced synchronized blocks on entryMap with read/write locks

[ofiterev]

Resolves #163

Hello,

while doing load tests I ran into a problem with threads blocking when reading from InMemoryDirectoryServer as it is using synchronized blocks, so I replaced them with read/write locks. Accepting this pull request would help me a lot since I will not need to rebuild each release :)

Thanks a lot, Valentin