gossiper::get_endpoint_state_for_endpoint() returns a copy of
endpoint_state, which we've seen can be very expensive.
This patch adds a similar function which returns a pointer instead,
and changes the call sites where using the pointer-returning variant
is deemed safe (the pointer neither escapes the function, nor crosses
any defer point).
Fixes#764
Signed-off-by: Duarte Nunes <duarte@scylladb.com>
ceebbe14cc