Implements reusing of rma buffers on the receiver side.

This is done by having the garbage collector return buffers no longer in use.

Note that there is no way to limit the amount of registered buffers, and that
clients must make sure to call the GC often enough.
Removes artificial limit in rma buffer sizes on the send size.
Adds comments in Pool.hs and improves indentation.
Removes obsolete file.
Protects Pool calls against asynchronous exceptions.
Merge pull request #6 from ps-labs/rma-reuse-draft

Rma reusing draft