Compute XID horizon for page level index vacuum on primary.

 Compute XID horizon for page level index vacuum on primary.
 
 Previously the xid horizon was only computed during WAL replay. That
 had two major problems:
 1) It relied on knowing what the table pointed to looks like. That was
    easy enough before the introducing of tableam (we knew it had to be
    heap, although some trickery around logging the heap relfilenodes
    was required). But to properly handle table AMs we need
    per-database catalog access to look up the AM handler, which
    recovery doesn't allow.
 2) Not knowing the xid horizon also makes it hard to support logical
    decoding on standbys. When on a catalog table, we need to be able
    to conflict with slots that have an xid horizon that's too old. But
    computing the horizon by visiting the heap only works once
    consistency is reached, but we always need to be able to detect
    conflicts.
 
 There's also a secondary problem, in that the current method performs
 redundant work on every standby. But that's counterbalanced by
 potentially computing the value when not necessary (either because
 there's no standby, or because there's no connected backends).
 
 Solve 1) and 2) by moving computation of the xid horizon to the
 primary and by involving tableam in the computation of the horizon.
 
 To address the potentially increased overhead, increase the efficiency
 of the xid horizon computation for heap by sorting the tids, and
 eliminating redundant buffer accesses. When prefetching is available,
 additionally perform prefetching of buffers.  As this is more of a
 maintenance task, rather than something routinely done in every read
 only query, we add an arbitrary 10 to the effective concurrency -
 thereby using IO concurrency, when not globally enabled.  That's
 possibly not the perfect formula, but seems good enough for now.
 
 Bumps WAL format, as latestRemovedXid is now part of the records, and
 the heap's relfilenode isn't anymore.
 
 Author: Andres Freund, Amit Khandekar, Robert Haas
 Reviewed-By: Robert Haas
 Discussion:
     https://postgr.es/m/20181212204154.nsxf3gzqv3gesl32@alap3.anarazel.de
     https://postgr.es/m/20181214014235.dal5ogljs3bmlq44@alap3.anarazel.de
     https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de