The primary goal here is to move all of the related page modifications
to a single section of code, in preparation for adding write-ahead
logging.  In passing, rename _hash_metapinit to _hash_init, since it
initializes more than just the metapage.

Amit Kapila.  The larger patch series of which this is a part has been
reviewed and tested by Álvaro Herrera, Ashutosh Sharma, Mark Kirkwood,
Jeff Janes, and Jesper Pedersen.