Originally GWBITMASK was capable of extending itself dynamically,
a functionality that in practice was never used, and that, in fact,
was broken.
As GWBITMASK is used for a single purpose inside MaxScale, its size
is now fixed at 256 bits, which is sufficient for 256 threads. Its
fixed size removes a few potential error situations that now then
do not need to be dealt with. And in principle is also more
performant.
In a separate change, bitmask_free will be renamed to bitmask_finish,
as contrary to free-functions in general, the function does not free
the argument, but only its data. The implementation will be empty,
but the function left in place for symmetry reasons (alloc/free,
init/finish).
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