Removed usage of pattern matching in PostgreSQL when not necessary (ref #970, fixes #974)

[leplatrem]

Ref #970, Fixes #974

r? @Natim

Note: this is a huge performance win.

[leplatrem]

Note: according to docs (and performance obtained locally), LIKE behaves exactly like = if the values do not contain % character

[leplatrem]

We could stick to LIKE and have more perf gain:

[dbname] # SELECT COUNT(*) FROM access_control_entries WHERE object_id SIMILAR TO '/buckets/[a-zA-Z\-_]+';
 count 
-------
     6
(1 row)

Time: 30,519 ms

[dbname] # SELECT COUNT(*) FROM access_control_entries WHERE object_id LIKE '/buckets/%' AND object_id NOT LIKE '/buckets/%/%';
 count 
-------
     6
(1 row)

Time: 4,435 ms