Get async cache store to aggregate or coalite multiple changes to a key.

So, assuming that the modification queue contains 3 changes for key=k1: k1=v1, k1=v2, k1=v3,
only write the last one rather than writing all 3 modifications.

This is something Oracle Coherence already does:
http://coherence.oracle.com/display/COH34UG/Read-Through%2C+Write-Through%2C+Write-Behind+and+Refresh-Ahead+Caching#Read-Through%2CWrite-Through%2CWrite-BehindandRefresh-AheadCaching-WriteBehindCache