snapshot too old, configured by time

As discussed when the "proof of concept" patch was submitted during
9.5 development, here is a version intended to be considered for
commit to 9.6, with the following changes:

1. It is configured using time rather than number of transactions.
Not only was there unanimous agreement here that this was better,
but the EDB customer who had requested this feature and who had
been testing it independently made the same request.

2. The "proof of concept" patch only supported heap and btree
checking; this supports all index types.

3. Documentation has been added.

4. Tests have been added.  They are currently somewhat minimal,
since this is using a whole new technique for testing from any
existing committed tests -- I wanted to make sure that this
approach to testing was OK with everyone before expanding it.  If
it is, I assume we will want to move some of the more generic
portions to a .pm file to make it available for other tests.

Basically, this patch aims to limit bloat when there are snapshots
that are kept registered for prolonged periods.  The immediate
reason for this is a customer application that keeps read-only
cursors against fairly static data open for prolonged periods, and
automatically fields SQLSTATE 72000 to re-open them if necessary.
When used, it should also provide some protections against extreme
bloat from forgotten "idle in transaction" connections which are
left holding a snapshot.

Once a snapshot reaches the age threshold, it can be terminated if
reads data modified after the snapshot was built.  It is expected
that useful ranges will normally be somewhere from a few hours to a
few days.

By default old_snapshot_threshold is set to -1, which disables the
new behavior.

The customer has been testing a preliminary version of this
time-based patch for several weeks, and is happy with the results
-- it is preventing bloat for them and not generating "snapshot too
old" errors at unexpected times.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company