Clear buffers under lock

[bitfaster]

The read and write buffer are both mp-sc (single consumer) - it is only valid for 1 thread to deque. Prior change switched from Clear allocating a new buffer to TryTake until empty.

Clearing the buffers must be protected by the maintenance lock, otherwise two threads can effectively dequeue - this situation is provoked by a test causing the build to hang intermittently.

[coveralls]

coverage: 99.178% (-0.06%) from 99.237%
when pulling 28121ca on users/alexpeck/fixclear
into 4201887 on main.