Use pg_rewind when target timeline was switched

Use pg_rewind when target timeline was switched

Allow pg_rewind to work when target timeline was switched. Now
user can return promoted standby to old master.

Target timeline history becomes a global variable. Index
in target timeline history is used in function interfaces instead of
specifying TLI directly. Thus, SimpleXLogPageRead() can easily start
reading XLOGs from next timeline when current timeline ends.

Author: Alexander Korotkov
Review: Michael Paquier