Replace the KnownAssignedXids hash table with a sorted-array data structure,

Replace the KnownAssignedXids hash table with a sorted-array data structure,
and be more tense about the locking requirements for it, to improve performance
in Hot Standby mode.  In passing fix a few bugs and improve a number of
comments in the existing HS code.

Simon Riggs, with some editorialization by Tom