This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Eliminate dependency on io_thread_get().

Required for Boost 1.61, which uses that overload in a BOOST_ASSERT().

Avoids a crash that occurs when NfsConnection::OnSocketReady()
dereferences itself before returning.

Reduce overhead for storing the key twice, and more overhead while
looking up the connection to remove it after a failure.

Necessary for "final" on gcc 4.6.

std::map::emplace() is only available from gcc 4.8 on.

More robust and cancellable.