Session-specific logging. Added functions
void session_enable_log(SESSION* ses, logfile_id_t id)
and
void session_disable_log(SESSION* ses, logfile_id_t id)

Which switch specific log type on/off if the log type in question is not generally enabled.

Each thread carries a thread-specific struct log_info_t which includes members for current session id and bitfield for enabled log types for the current session. That information is checked before actual log write functions are called.

Each file where session-specific logging is used, must include the following exports:

/** Defined in log_manager.cc */
extern int            lm_enabled_logfiles_bitmask;
extern size_t         log_ses_count[];
extern __thread log_info_t tls_log_info;
Merge branch 'develop' of https://github.com/mariadb-corporation/MaxScale into develop