20637: Move the prefetch thread pool to KeepClient

20637: Move the prefetch thread pool to KeepClient

This avoids the problem of every Collection with its own BlockManager
creating its own prefetch thread pool, which becomes a resource leak
when reading files from 1000s of separate Collection objects.

The 'put' thread pool remains with the BlockManager but it now stops
the put threads on 'BlockManager.commit_all'.  This is because this
method always flushes pending blocks anyway, and is called before the
collection record is written to the API server -- so we can assume
we've just finished a batch of writes to that collection, and might
not need the put thread pool any more, and if we do, we can just make
a new one.

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <peter.amstutz@curii.com>