RDS: Change send lock from a mutex to a spinlock

This change allows us to call rds_send_xmit() from a tasklet,
which is crucial to our new operating model.

* Change c_send_lock to a spinlock
* Update stats fields "sem_" to "_lock"
* Remove unneeded rds_conn_is_sending()

About locking between shutdown and send -- send checks if the
connection is up. Shutdown puts the connection into
DISCONNECTING. After this, all threads entering send will exit
immediately. However, a thread could be *in* send_xmit(), so
shutdown acquires the c_send_lock to ensure everyone is out
before proceeding with connection shutdown.

Signed-off-by: Andy Grover <andy.grover@oracle.com>