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.