Revert "Revert "Add a bufio buffer to rpc/bsonrpc.""

This reverts commit cdf516d52843e22b9e55e68ad035758fc8127be9.

Putting back performance improvements to rpc/bsonrpc, this was not the
cause of issues that are being observed.

Conflicts:
	rpc/bsonrpc/bsoncoders.go