avoid cache avalanche

[xpineal]

At the beginning of client start, there is no table region info in cache.
If there are many go routine to access a table at same time, they will miss cache and send request package to get table region info.
In this case, too many duplicated requests should be avoid.

[dethi]

This looks similar to #164, not in the implementation but in what it tries to do.

[tsuna]

Discussed this with @dethi today and after looking at data on our clusters with @aaronbee we decided to add an option to the client to perform a cache warm up during initialization by preloading all entries from meta for tables of interest.