Move pogreb db writes into a single goroutine

Avoid lock contention on the pogreb db rwmutex by sending db write
requests over a buffered channel to a single goroutine that handles
the calls to pogreb.DB.Put().

Reduces full analysis time with incremental analysis enabled from
207 seconds to 184 seconds.

Test: context_test.go
Change-Id: I41d93961572a818ece2cc4fd3d40e622d7c0623b