Add $basedir/lib/plugin to the search paths for semisync plugins.

The semisync plugin library names on Unix like systems were prefixed with
'lib', which did not follow the conventions.

Fix the problem by removing the 'lib' prefix on Unix systems.

rpl_semi_sync_master_wait_sessions was reset by FLUSH STATUS,
which could cause the master fail to wake up waiting sessions and
result in master timeout waiting for slave reply.

rpl_semi_sync_master_wait_session should not be reset, this 
problem is fixed by this patch.

Remove functions that no longer needed
Fix warning suppressions

Add an option to control whether the master should keep waiting
until timeout when it detected that there is no semi-sync slave
available.

The bool option 'rpl_semi_sync_master_wait_no_slave' is 1 by
defalt, and will keep waiting until timeout. When set to 0, the
master will switch to asynchronous replication immediately when
no semi-sync slave is available.

Semi-sync status were not reset by FLUSH STATUS, this was because
all semi-sync status variables are defined as SHOW_FUNC and FLUSH
STATUS could only reset SHOW_LONG type variables.

This problem is fixed by change all status variables that should
be reset by FLUSH STATUS from SHOW_FUNC to SHOW_LONG.

After the fix, the following status variables will be reset by
FLUSH STATUS:
  Rpl_semi_sync_master_yes_tx
  Rpl_semi_sync_master_no_tx

Note: normally, FLUSH STATUS itself will be written into binlog
and be replicated, so after FLUSH STATS, one of
  Rpl_semi_sync_master_yes_tx
  Rpl_semi_sync_master_no_tx
can be 1 dependent on the semi-sync status. So it's recommended
to use FLUSH NO_WRITE_TO_BINLOG STATUS to avoid this.

Errors when send reply to master should never cause the IO thread
to stop, because master can fall back to async replication if it
does not get reply from slave.

The problem is fixed by deliberately ignoring the return value of
slaveReply.

Semi-sync uses an extra connection from slave to master to send
replies, this is a normal client connection, and used a normal
SET query to set the reply information on master, which is visible
to user and may cause some confusion and complaining.

This problem is fixed by using the method of sending reply by
using the same connection that is used by master dump thread to
send binlog to slave. Since now the semi-sync plugins are integrated
with the server code, it is not a problem to use the internal net
interfaces to do this.

The master dump thread will mark the event requires a reply and
wait for the reply when the event just sent is the last event
of a transaction and semi-sync status is ON; And the slave will
send a reply to master when it received such an event that requires
a reply.

On sparc, semisync master/slave status is always showed as OFF, this
is fixed by change rpl_semisync_master/slave_status variables from
long to char.

Fix mtr semisync plugin option paths

Backporting BUG#44058 BUG#42244 BUG#45672 BUG#45673
Backporting BUG#45819 BUG#45973 BUG#39012

Extract substr into local variable

with the newer pb2 testing environments