lua: convert flowvar functions to lib

lua: convert flowvar functions to lib

New Lua lib, "suricata.flowvar" for working with flowvars from Lua.

Replaces functions:
- SCFlowvarGet (and ScFlowvarGet)
- SCFlowvarSet (and SCFlowvarSet)

Of note, the DetectLuaData has been made available to the init and
thread_init methods, instead of just the match. This is due to an
issue that if a flow variable is not registered in init, it will not
be logged, registering in thread_init is too late.

Ticket: #7486