Buffer DB updates of playmode to reduce writes.
Do not persist StreamingController TRACKWAIT state

Presisting StreamingController state is only done so that it can be
restored in SlimService in another instance. The TRACKWAIT state is used
when asynchronous callback of a closure is expected and such state cannot
be meaningfully restored.

Rely on subsequent event (possibly resent by player after reconnect)
to trigger next action

Change-Id: I2cd85e18b34302589e39c4366e1be66a8cd9163b
Merge branch 'public/7.7' into hotfix/delayed-playmode-write
Simplify buffering
Merge branch 'hotfix/delayed-playmode-write' into public/7.7