Synchronized Scan patch

This is the Synchronized Scan patch.

This patch only implements the simplest Synchronized Scan features; I
removed some of the other features for simplicity of testing.

This patch:
(1) allocates shared memory to store a table of (relation oid,
blocknumber) pairs.
(2) When fetching a new page in a scan, it hashes the Oid of the
relation and stores a hint entry in the shared memory table
(2) When starting a new scan, it reads the shared memory table for the
hint and starts the scan at that location (which is, hopefully, near an
already-existing scan).

This isn't complete. I still plan to add some features and do a lot of
testing, depending on feedback. I am posting it here so that anyone can
test it, and so when I show results I can refer to this specific patch.

Regards,
Jeff Davis