Trim THD by moving out fields specific to system threads

The class THD has a number of fields that are used only in special system
threads (eg. replication SQL/worker threads), yet take up space (and CPU
cache) for all threads.

We should intruduce a THD::system_thread_info, which is a union of pointers to
different structures used in different system threads. So that only a single
pointer needs to be added to the THD to be able to handle all extra
information needed by any special-purpose thread:

  union {

    rpl_io_thread_info *rpl_io_info;

    rpl_sql_thread_info *rpl_sql_info;

    // ...

  } system_thread_info;

Non-exhaustive list of candidates to be moved into this union:

• rpl_filter
• rgi_slave
• is_slave_error
• slave_expected_error
• slave_net

In addition, it should be enough to have a single field,
eg. THD::system_thread, that identifies the different system threads and thus
which of the pointers in the system_thread_info union are valid. This should
replace fields such as THD::slave_thread.