scm: fixes and optimizations for connection scaling

scm: fixes and optimizations for connection scaling

Prioritize accepts on listen ports via FD_READ
process the accepts ahead of other work to avoid
socket half_connection (SYN_RECV) stalls.

Fix dapl_poll to return DAPL_FD_ERROR on
all event error types.

Add new state for socket released, but CR
not yet destroyed. This enables scm to release
the socket resources immediately after exchanging
all QP information. Also, add state to str call.

Only add the CR reference to the EP if it is
RC type. UD has multiple CR's per EP so when
a UD EP disconnect_clean was called, from a
timeout, it destroyed the wrong CR.

Signed-off-by: Arlin Davis <arlin.r.davis@intel.com>