MEDIUM: hlua: introduce tune.lua.burst-timeout

MEDIUM: hlua: introduce tune.lua.burst-timeout

The "burst" execution timeout applies to any Lua handler.
If the handler fails to finish or yield before timeout is reached,
handler will be aborted to prevent thread contention, to prevent
traffic from not being served for too long, and ultimately to prevent
the process from crashing because of the watchdog kicking in.

Default value is 1000ms.
Combined with forced-yield default value of 10000 lua instructions, it
should be high enough to prevent any existing script breakage, while
still being able to catch slow lua converters or sample fetches
doing thread contention and risking the process stability.

Setting value to 0 completely bypasses this check. (not recommended but
could be required to restore original behavior if this feature breaks
existing setups somehow...)

No backport needed, although it could be used to prevent watchdog crashes
due to poorly coded (slow/cpu consuming) lua sample fetches/converters.