Revert RDMA read fragmentation commits for now

This commit undoes the implementation of RDMA read fragmentation as it's broken
in the context of threaded CQ handling at this time. Issue #204 will look at this.

Revert "Implement RDMA read fragmentation"
This reverts commit 86540e32.

Revert "Add support for fragments in rdma completion context"
This reverts commit 151b7b30.

Revert "Check maximum supported transfer size of endpoints, raise error"
This reverts commit 7b5f3edf.