PG: clear want_acting when we leave Primary

This is somewhat annoying actually.  Intuitively we want to
clear_primary_state when we leave primary, but when we restart
peering due to a change in prior set status, we can't afford
to forget most of our peering state.  want_acting, on the
other hand, should never persist across peering attempts.
In fact, in the future, want_acting should be pulled into
the Primary state structure.

Fixes: #3904
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>