MDEV-7257: Dump Thread Enhancements

Make the binlog dump threads not need to take LOCK_log while sending
binlog events to slave. Instead, a new LOCK_binlog_end_pos is used
just to coordinate tracking the current end-of-log.

This is a pre-requisite for MDEV-162, "Enhanced semisync
replication". It should also help reduce the contention on LOCK_log on
a busy master.

Also does some much-needed refactoring/cleanup of the related code in
the binlog dump thread.
MDEV-162 Enhanced semisync replication

Implement --semi-sync-master-wait-point=AFTER_SYNC|AFTER_COMMIT.

When AFTER_SYNC, the semi-sync wait will be done earlier, before the storage
engine commit rather than after. This means that a transaction will not be
visible on the master until at least one slave has received it.
MDEV-7053 : WSREP_STATUS & WSREP_MEMBERSHIP I_S tables

Addendum : Renamed plugin .so file.
MDEV-7277 Server crashes on creating/opening tables on Windows debug build.
        The srid variable was used uninitialised when the field wasn't GIS.
        Only problem is that it makes the debugger unhappy. Still added
        the initialization.
MDEV-7364 - mysqld --help --verbose prints random values for "debug"

getopt value pointer of "debug" variable was pointing to incorrect address:
&global_system_variables. Runtime statements like SHOW VARIABLES materialize
value from DBUG structures on demand, so they never access getopt value pointer.
But mysqld --help --verbose loaded this value from &global_system_variables.

Remove "debug" variable from mysqld --help --verbose output by setting value
pointer to NULL.
MDEV-7053 - WSREP_STATUS & WSREP_MEMBERSHIP I_S tables

wsrep_status was renamed to wsrep_info. Adjusted deb build scripts accordingly.
MDEV-7273 - 10.1 fails to start up during tc_log initializations on PPC64

log-tc-size is 24K by default. Page size is 64K on PPC64. But log-tc-size
must be at least 3 x page size. This is enforced by TC_LOG_MMAP::open()
with a comment: to guarantee non-empty pool.

This all makes server not startable in default configuration on PPC64.

Autosize log-tc-size, so that it's min value= page size * 3, default
value= page size * 6, block size= page size.
Add fix for MDEV-7369

See also https://mariadb.atlassian.net/browse/MDEV-7369
Merge pull request #14 from James-TR/MDEV-7369

Add fix for MDEV-7369
MDEV-7262: innodb.innodb-mdev7046 fail on BuildBot
lf_hash changes, in lfind()

casts, etc

real changes are:
* remove one retry, it is enough to check for DELETED
  after the key is read
* advance 'head' pointer when we see a dummy node to have
  shorter retries
lf_hash_iterate() function
MDEV-7324 - Lock-free hash for table definition cache