Refresh good slaves count when setting slave state as online.
New INFO field "min_slaves_good_slaves".

When min-slaves-to-write feature is active, this field reports the
number of slaves considered good (online state, lag within the specified
range).
Refresh good slaves count after CONFIG SET min-slaves-...

This way just after the CONFIG SET enabling the min-slaves feature it is
possible to write to the database without delays.
Test: avoid a false positive in min-slaves test.
CONFIG SET: accept slave-priority zero, it is valid.