hash: Refactor and clean up bucket split code.

hash: Refactor and clean up bucket split code.

As with commit 30df93f698d016d086e8961aa6c6076b37ea0ef4 and commit
b0f18cb77f50a54e997d857d592f6a511617f52c, the goal here is to move all
of the related page modifications to a single section of code, in
preparation for adding write-ahead logging.

Amit Kapila, with slight changes by me.  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.