SimpleMessenger: Don't grab the lock when sending messages if we don't have to

This patch makes the common case no longer take a global lock when submitting a Message, thereby scaling better. Hurray!

I ran it through a rados suite and it was largely clean (http://qa-proxy.ceph.com/teuthology/gregf-2014-04-22_14:24:11-rados-wip-messenger-locking-testing-basic-plana/209595/). Several tests failed on an uninitialized rwstate, which was an upstream issue since fixed, and one test failed on the PG ref on shutdown bug.
I was unable to identify the actual issue with one test: http://qa-proxy.ceph.com/teuthology/gregf-2014-04-22_14:24:11-rados-wip-messenger-locking-testing-basic-plana/209595; it was somewhere in cephtool/test.sh and did not leave any core dumps nor an identifiable log.