librbd: switch the IO path to utilize the neorados API

librbd: switch the IO path to utilize the neorados API

IO operations to the cluster are now dispatched via the neorados
API which allows multiple threads to be utilized for processing
incoming and outgoing IO.

This also involves switching from a map for tracking sparse extents
to a vector of pairs since that matches the new API for sparse
read operations.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>