pgsql: Get rid of WALBufMappingLock

Get rid of WALBufMappingLock

Allow multiple backends to initialize WAL buffers concurrently.  This way
`MemSet((char *) NewPage, 0, XLOG_BLCKSZ);` can run in parallel without
taking a single LWLock in exclusive mode.

The new algorithm works as follows:
 * reserve a page for initialization using XLogCtl->InitializeReserved,
 * ensure the page is written out,
 * once the page is initialized, try to advance XLogCtl->InitializedUpTo and
   signal to waiters using XLogCtl->InitializedUpToCondVar condition
   variable,
 * repeat previous steps until we reserve initialization up to the target
   WAL position,
 * wait until concurrent initialization finishes using a
   XLogCtl->InitializedUpToCondVar.

Now, multiple backends can, in parallel, concurrently reserve pages,
initialize them, and advance XLogCtl->InitializedUpTo to point to the latest
initialized page.

Author: Yura Sokolov <y.sokolov@postgrespro.ru>
Co-authored-by: Alexander Korotkov <aekorotkov@gmail.com>
Reviewed-by: Pavel Borisov <pashkin.elfe@gmail.com>

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/6a2275b8953a4462d44daf001bdd60b3d48f0946

Modified Files
--------------
src/backend/access/transam/xlog.c               | 176 +++++++++++++++++-------
src/backend/utils/activity/wait_event_names.txt |   2 +-
src/include/storage/lwlocklist.h                |   2 +-
3 files changed, 132 insertions(+), 48 deletions(-)