* Add. Handler for SUBSCRIBE method for call flow application.
Usage:
1. Run form fs_cli `luarun call_flow_subscribe`
2. Create new call flow extension and set feature code to `flow+<EXTENSION>`(e.g. `flow+401`).
3. Set on the phone BLF key to `flow+401`
This code based on `mod_valet_parking`.
* Add. prevent running 2 copy of script.
Remove some unused vars and simplify implementation.
* Fix. Use correct protocol for send event.
* Fix. Do escape SQL arguments
* Fix. escape `+` sign in call flow extension.
Alexey Melnichuk