Before this patch, semisync assumed transactions running in parallel
can not be larger than max_connections, but this is not true when
the event scheduler is executing events, and cause semisync run out
of preallocated transaction nodes.

Fix the problem by allocating transaction nodes dynamically.

This patch also fixed a possible deadlock when running UNINSTALL
PLUGIN rpl_semi_sync_master and updating in parallel. Fixed by
releasing the internal Delegate lock before unlock the plugins.

mysql-test/suite/rpl/t/rpl_semi_sync_event.test:
  Add test case for bug#49020
plugin/semisync/semisync_master.cc:
  Allocating TranxNode dynamically
plugin/semisync/semisync_master.h:
  Allocating TranxNode dynamically
sql/rpl_handler.cc:
  Unlock plugins after we have released the Delegate lock to avoid possible deadlock when uninstalling semisync master plugin and doing update in parallel.