Added check for session state before calling router->sessionClose
MAX-157. Added support for hints in rwsplit router.
buffer.c: added memory release for hint of a GWBUF
hint.c: added bool hint_exists()
hint.h: added placeholder for hint type HINT_ROUTE_TO_ALL which doesn't have implementation yet.
filter/Makefile: fixed dependency issue
hintparser.c: added const char* token_get_keyword(), hint_parser:added NULL check, hint_next_token: fixed off-by-one bug
readwritesplit.h: added bitfield for hints' use, which includes route targets and flag for case where user hinted to route to named backend server.
readwritesplit.c: added function route_target_t get_route_target() for resolving route target based on 1) query type (from query_classifier) 2) transaction state (active/not) and 3) hints. Modified get_dcb, which is called in routeQuery to provide pointer to correct backend DCB. Now get_dcb also takes server unique name as a parameter if such a hint was found. for hints' use, which includes  enter the commit message for your changes.
config.h: Added enumerated types MAX_RLAG_AVAILABLE and MAX_RLAG_UNDEFINED to be used instead of magic numbers as max_slave_replication_lag values.
readwritesplit.h: Added TARGET_RLAG_MAX to the list of hint targets. It is used in case of hint max_slave_replication_lag=<value>
readwritesplit.c: get_dcb is called when readwritesplit router chooses target backend for a query. Backend is selected from those the router has connection to. get_dcb now takes additional parameters, name and max_rlag. Name is used if there is a hint route to server <server name>. max_rlag is used for hint max_slave_replication_lag=<value> or if it doesn't exist, then for configure parameter max_slave_replication_lag=<configured value> or if both are absent, ignored.
Applying changes made in commit fa1f9ec13113e00e65f2425b080cfb0d102b0872 to this branch.
readwritesplit.c: corrected max_replication_lag with max_slave_replication_lag in hint parameter comparison functions.
Merge remote-tracking branch 'maxscale/MAX-157' into hint_filter_tests

Conflicts:
	server/core/modutil.c
rwsplit hint mechanism tests, input file is 'hints.txt' and hints and expected values are separated by colons