mgr/dashboard: RGW buckets async validator performance enhancement

mgr/dashboard: RGW buckets async validator performance enhancement

The rgw bucket creation form has the Name field which have an async
validator. The validator calls all the bucket name and check if the
entered name is unique or not. This happens on every keystroke. So if
100 or more buckets are there, then the async validation can be real
    slow and causes misvalidations in different fields.

I changed the validation logic and did some cleanups to improve the
performance of the async validation.

Fixes: https://tracker.ceph.com/issues/50514
Signed-off-by: Nizamudeen A <nia@redhat.com>