CHANGE write_concurrent ets with no gen server serialization

Please don't merge this yet, cause its performance depends on ets write_concurrent performance. According to this paper by Kostis Sagonas, write concurrency doesn't have a negative impact on lookups, just a smaller increase in the memory space consumed by your tables. However, serializing write operations through a gen_server was having a negative impact on the parallelism/scalability of erl_cache, since erl_cache_server could potentially become a bottleneck.

http://release-project.softlab.ntua.gr/documents/D2.3.pdf