Delay reading timeline history file until it's fetched from master.

 Delay reading timeline history file until it's fetched from 
 
 Streaming replication can fetch any missing timeline history files from the
 master, but recovery would read the timeline history file for the target
 timeline before reading the checkpoint record, and before walreceiver has
 had a chance to fetch it from the master. Delay reading it, and the sanity
 checks involving timeline history, until after reading the checkpoint
 record.
 
 There is at least one scenario where this makes a difference: if you take
 a base backup from a standby server right after a timeline switch, the
 WAL segment containing the initial checkpoint record will begin with an
 older timeline ID. Without the timeline history file, recovering that file
 will fail as the older timeline ID is not recognized to be an ancestor of
 the target timeline. If you try to recover from such a backup, using only
 streaming replication to fetch the WAL, this patch is required for that to
 work.