Listed and described shortly max_slave_connections and server_options=slave_selection_criteria parameters.
Removed unnecessary SERVER pointer from dcb.c:dcb_call_foreach(), changed declaration and call accordingly.
Added mechanism for choosing slave for a query based on the current load in all connected slaves. Counting operations is not correctly done here. Reading values and choosing accordingly is done.

Fixed several things in session command reply processing.
Enabled set and clear the flag BREF_WAITING_RESULT. It is actually a counter which is modified with atomic operations only. Setting and clearing BREF_WAITING_RESULT also includes atomic increment and decrement of corresponding backend server's current operations counter, backend_server->stats.n_current_ops.
Clean up
Added status bit BREF_QUERY_ACTIVE to indicate if a query, other than session command, sent to backend for execution. The flag is cleared when the first packet belonging to the response arrives.
The flag is part of the active operation counting, which is utilized in load balancing. The active operation count per backend is used by default as criteria when router chooses to which backend a query should be routed.
Merge branch 'develop' into MAX-167

Conflicts:
	server/modules/monitor/mysql_mon.c
	server/modules/routing/readwritesplit/readwritesplit.c
Merge pull request #7 from skysql/MAX-167

Max 167