Flexible bulk_get using postgres arrays as WHERE id = ANY($1)

Rather than implementing bulk_get queries as hard-coded prepared
queries via IN for 1, 2, 3, 4, or 5 items, we now use a Postgres array
as the query parameter and the ANY() function.

Accoding to this post [1], the efficiency of the operation should be
equivalent to the IN approach.

[1]: http://www.postgresql.org/message-id/5373.1158351561@sss.pgh.pa.us