Allow passing a memory limit for bins

[rbino]

Currently the only variables controlling a bin lifetime are LIVE_BIN_INACTIVITY_TIMEOUT and LIVE_BIN_MAX_REQUESTS. Those two don't protect from the fact that a bin could receive very big requests, possibly bringing down the whole BEAM if it gets killed by the OOM.

Another env variable must be added to handle this, allowing to set the max number of bytes a bin can receive