PG: use int64_t for pool id in PGPool

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG: update PGPool::name in PGPool::update

Fixes: #4471
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #168 from athanatos/wip_4471

Wip 4471

Reviewed-by: Sage Weil <sage@inktank.com>