Initial implementation of the support for multiple slaves in read/write split router session. Omits configuration changes.
Added new parameter RW Split Router.max_slave_connections=[<int>|<int>%] which specifies the maximum number of slaves which read/write split router connects in each routing session.

Parameter it read from config file to CONFIG_CONTEXT's parameter list. It is qualified in service.c:service_set_slave_conn_limit and if qualified, the qualified integer value and the value type are copied to the CONFIG_PARAMETER structure.

This CONFIG_PARAMETER struct is cloned (=copied to different memory area) and linked to RW Split SERVICE struct.

When RW Split router_instance is created in readwritesplit.c:createInstance, the value is copied to (new) rwsplit_config_t structure from SERVICE's parameter list.

When new routing session is created in readwritesplit.c:newSession, the rwsplit_config_t structure is copied to ROUTER_CLIENT_SES struct and the actual max_nslaves value is calculated from the config value (if percentage is used).

Tests and many error handling branches are missing but functionality seems to be working.
Fixed two bugs of which one was older.
1. in query_classifier.cc autocommit_enabled, and transaction_active variables maintained their values across different sessions. Now those values are stored in each router_client_ses object.
2. As a part of implementation of MAX-95 session variables were added to BACKEND struct which is shared with all sessions using the SERVICE which the particular BACKEND serves. Now each router_client_ses object has a backend reference struct which includes pointer to BACKEND, DCB and to session command cursor.

Added test - set_autocommit_disabled.sql, test_after_autocommit_disabled.sql - to check that session variable is discarded when session where it belongs terminates.
Removed this because it is deprecated.
log_manager.cc fixed memory leak, block buffer mutex names weren't freed.
query_classifier.cc use of uninitialized value in skygw_stmt_causes_implicit_commit
config.c crashed if module load failed, use of unitialized value
load_utils.c pretty-printed error
service.c use of uninitialized value in service_add_qualified_param
modules.h function prototype
readwritesplit.c memory leaks
Multiple slave connections for read/write split router.

Merge branch 'MAX-95' into develop