Support for mysql session variable commands, for example, SET AUTOCOMMIT=0
Session commands are identified by query clasisfier, and added to the session command property list in router client session object.
Session commands are then executed in existing backend servers but only one of them will reply to client.
Added support for session commands to readwrite split router.
Added support for multi-statement packets.

This is an intermediate commit to save work. Code is not cleaned and there are debug prints and prototypes to be removed.
Router has now capability value which currently tells whether router session expects stream or individual, complete statements. With read con
nection router stream is used and with read/write split router individual statements are passed to router.
Added new function to ROUTER_OBJECT : uint8_t (*getCapabilities)(ROUTER *instance, void* router_session); which is implemented in every route
r.

Added support for multi-statement packets in rwsplit router. In other words, if network packet includes multiple mysql statements, they are separated and passed to router one by one.

Multi-packet statements (those which exceeds network packet boundaries) are _not_ supported yet.
Extended session command support to cover COM_CHANGE_USER, and COM_INIT_DB.
This implementation doesn't guarantee execution order between session commands and queries
if other backend server lags behind in session command execution.

In poll.c : moved processing of EPOLLERR and EPOLLHUP after processing of EPOLLIN and EPOLLOUT.
This ensures that COM_QUIT messages are read and routed forward before signals arrive (from local client/backend).
readwritesplit.c : router is changed so that it guarantees to keep the execution order of session commands and queries when they are routed to backend servers. In the same way it maintains the order of response packets and discards duplicate responses.
For each session command a sescmd property is created and added to the end of list of session commands. List is owned by router client session and it includes all session commands from the beginning of router session.
Router maintains an individual session command cursor for each backend. A cursor refers to the first session command which the corresponding backend server haven't yet responded yet.
When response message arrives at any time from backend, first it is checked whether backend's cursor is active. Cursor is active if a session command is routed to backend and the backend haven't responded to it yet. If cursor is active, next it is checked whether the current session command property has been responded by other backend. If both are true, then response message is sent to client as is. If session command response is routed to client already, the arrived response is discarded.
Fixed bug in how response messages from backends were handled. Now messages are examined if session command cursor is active
. That means that (at least) the first packet  in the message is response to session command. Then if the response is alread
y forwarded to client it is discarded. Otherwise it is routed to client and the command is marked as responded so that the o
ther backend knows to discard its duplicate response.
Merge branch 'SESvars' of https://github.com/skysql/MaxScale into SESvars

Conflicts:
	server/core/dcb.c
	server/core/poll.c
	server/modules/include/mysql_client_server_protocol.h
	server/modules/routing/readwritesplit/readwritesplit.c
Merging SESvars with develop.