Hibernate rexi_buffer when becoming idle

The rexi_buffer gen_server can hold onto quite a bit of RAM during idle
operation. This just checks when we're going back to the idle state and
hibernates until the next message arrives. This ensures that we run
garbage collection before sitting idle.

BugzId: 27672
Merge pull request #23 from cloudant/27672-rexi-buffer-hibernation

Hibernate rexi_buffer when becoming idle