Teach xlogreader to follow timeline switches

 Teach xlogreader to follow timeline switches
 
 Uses page-based mechanism to ensure we’re using the correct timeline.
 
 Tests are included to exercise the functionality using a cold disk-level copy
 of the master that's started up as a replica with slots intact, but the
 intended use of the functionality is with later features.
 
 Craig Ringer, reviewed by Simon Riggs and Andres Freund