Prevent walsummarizer from getting stuck at a timeline switch.

Prevent walsummarizer from getting stuck at a timeline switch.

As previously coded, walsummarizer only wants to read WAL from a file
where the TimeLineID in the filename exactly matches the TimeLineID being
summarized. But in some cases, when a timeline switch occurs, the WAL file
from the old timeline is not archived, because it's never completely
filled, so the only way to obtain the contents of that last partial
segment is to read from the first segment on the new timeline. Teach
WAL summarizer to do that, and add a test case to make sure that it
works.

Reported-by: Nick Ivanov <nick.ivanov@enterprisedb.com>
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Tested-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Srinath Reddy Sadipiralla <srinath2133@gmail.com>
Reviewed-by: Zhijie Hou <houzj.fnst@fujitsu.com>
Reviewed-by: Thom Brown <thom@linux.com>
Discussion: http://postgr.es/m/CA+Tgmobr27GpKDZx3_ezW2+C5_g18i+jSK3sGF_cR-_ESv5N5A@mail.gmail.com
Backpatch-through: 17