Miscellaneous code review changes from 5.7 work

Minor issues discovered in 5.7 porting and fixed there:
- THD::extra_port field is unused
- A duplicated code fragment in open_tables:

        if (need_prelocking && ! thd->lex->requires_prelocking())
          thd->lex->mark_as_requiring_prelocking(save_query_tables_last);

        if (need_prelocking && ! *start)
          *start= thd->lex->query_tables;

        if (need_prelocking && ! thd->lex->requires_prelocking())
          thd->lex->mark_as_requiring_prelocking(save_query_tables_last);

        if (need_prelocking && ! *start)
          *start= thd->lex->query_tables;
- Leftover #define EXTENDED_FOR_COMMIT_ORDERED
- Unused LOCK_stats mutex
- The tail of percona_heap_blob.test tests MyISAM not MEMORY as intended (as a new connection takes the system default_storage_engine value)
- hp_reallocate_chunkset has one plain return instead of DBUG_RETURN
- "--replace_column 3 CONNECTED_TIME 4 BUSY_TIME 5 CPU_TIME
SHOW CLIENT_STATISTICS;" in percona_userstat_test is one-off for replaced column names
- main.percona_processlist_pool_of_threads needs --source include/linux.inc
- threadpool_net_before_header_psi_noop, threadpool_net_after_header_psi can be made static
- mysqldump.c:skipped_keys_list can be made static
- QUERY_PROFILE::get_profiling can be made const
- PROFILING::enabled_getrusage can be made const
- PROFILING::print_current can be made const
- sys_vars.cc:void update_log_slow_verbosity(ulonglong* value_ptr) can be made static
- void threadpool_init_net_server_extension(THD *thd) can be made static
- int io_poll_associate_fd(int pollfd, int fd, void *data) can be made static
- int io_poll_start_read(int pollfd, int fd, void *data) can be made static
- plugin/percona-pam-for-mysql/.bzrignore can be deleted
- rpl.pseudo_server_id* testcases can have --source include/have_innodb.inc