pg_pool_t: enable FLAG_HASHPSPOOL by default

Fixes: #5160
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
doc: Updated index for newer terms.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Updated RGW Quickstart.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Updated rgw.conf example.

fixes: #4608

Signed-off-by: John Wilkins <john.wilkins@inktank.com>