region: Generalize the mutex of the region.Info
region: Generalize the mutex of the region.Info

It's going to protect more than just the `available' chan.

Change-Id: I4b4f2a9b6bc0f2f713a3afdcae1767d8081f5df6

tsuna/gohbase,dougklein/gohbase,dougklein/gohbase