msg/Pipe: make STANDBY behavior optional

msg/Pipe: make STANDBY behavior optional

In particular, lossless_peers should use STANDBY, but lossless_clients
should reconnect immediately since they are already doing their own session
management.

Specifically, this fixes the problem where the Client tries to open a
connection to the MDS and faults after delivering its OPEN_SESSION message
but before it gets the reply: the session isn't open yet, so it isn't
pinging.  It could, but it is simpler and faster to make the msgr layer
keep the connection open instead of waiting for a periodic keepalive.

Fixes: #2824
Signed-off-by: Sage Weil <sage@inktank.com>