Branch: b1_4

Branch: b1_4
Don't clobber last_rcvd upon mds_client_add() failure.  This was caused by
bad error handling in mds_connect() not setting med_mcd = NULL after freeing
it, and then zeroing the "client data" at med_off = 0.  Includes test.
This commit adds some safety checks to {mds,filter}_finish_transno()
and reconciles the differences between the mds and obdfilter last_rcvd
functions.
b=6086
r=nathan