A few patches to clarify snapshot management, part 2

This is a continuation of the earlier thread with the same subject [1],
and related to the CSN work [2].

I'm pretty happy patches 0001-0005. They make the snapshot management
more clear in many ways:

Patch 0001: Use a proper type for RestoreTransactionSnapshot's PGPROC arg

Minor cleanup, independent of the rest of the patches

Patch 0002: Split SnapshotData into separate structs for each kind of
snapshot

This implements the long-standing TODO and splits SnapshotData up into
multiple structs. This makes it more clear which fields are used with
which kind of snapshot. For example, we now have properly named fields
for the XID arrays in historic snapshots. Previously, they abused the
'xip' and 'subxip' arrays to mean something different than what they
mean in regular MVCC snapshots.

This introduces some new casts between Snapshots and the new
MVCCSnapshots. I struggled to decide which functions should use the new
MVCCSnapshot type and which should continue to use Snapshot. It's a
balancing act between code churn and where we want to have casts. For
example, PushActiveSnapshot() takes a Snapshot argument, but assumes
that it's an MVCC snapshot, so it really should take MVCCSnapshot. But
most of its callers have a Snapshot rather than MVCCSnapshot. Another
example: the executor assumes that it's dealing with MVCC snapshots, so
it would be appropriate to use MVCCSnapshot for EState->es_snapshot. But
it'd cause a lot code churn and casts elsewhere. I think the patch is a
reasonable middle ground.

Patch 0003: Simplify historic snapshot refcounting

Little refactoring in logical decoding's snapshot tracking.

Patch 0004: Add an explicit 'valid' flag to MVCCSnapshotData

Makes it more clear when the "static" snapshots returned by
GetTransactionSnapshot(), GetLatestSnapshot() and GetCatalogSnapshot()
are valid.

Patch 0005: Replace static snapshot pointers with the 'valid' flags

Refactor snapmgr.c a little, taking advantage of the new 'valid' flags

The rest of the patches are less polished, but please take a look. The
idea is to split MVCCSnapshot further into a "shared" part that contains
the xmin, xmax and the 'xip' array, and an outer shell that contains the
mutable 'curcid' field. This allows reusing the shared part in more
cases, avoiding copying, although I'm not sure what would be a practical
scenario where it'd make a performance difference. It becomes more
important with the CSN patch, however, because it adds a cache to the
shared snapshot struct, which can potentially grow very large.

[1]
https://www.postgresql.org/message-id/7c56f180-b9e1-481e-8c1d-efa63de3ecbb%40iki.fi
[2]
https://www.postgresql.org/message-id/80f254d3-8ee9-4cde-a7e3-ee99998154da%40iki.fi

- Heikki