crimson: introduce and use repop stage

crimson: introduce and use repop stage

Repops previously used PGPipeline::await_map.  This is actually
important as we need them to be processed in order.  However, using
await_map was confusing and using a single exclusive stage is decidedly
unoptimal as we could allow pipelineing on write commit.  For now, move
them over to their own pipeline stage so we can remove the PGPipeline
struct entirely.  Later, we'll improve replica write pipelining for
better replica-side write concurrency.

Signed-off-by: Samuel Just <sjust@redhat.com>