This allows for tunability of the following behaviors:
* Minimum flowbuffer capacity to use when parsing a new unit
* Threshold at which flowbuffer capacity is contracted back to the
minimum after parsing a complete unit and before parsing the next
* Maximum flowbuffer capacity to allow when parsing a given unit
Failed flowbuffer allocations due to reaching maximum capacity or any
other reason now throw ExceptionFlowBufferAlloc.
before anything else.
Internally, this function compiles all regular expressions, avoiding
to that inside the regexp constructore. The code is a bit hackish due
to the way the regexp code depends on the Bro header.
