expose resources

[aeb-dev]

This PR exposes _maxAllocatedResources, _allocatedResources and an extra property hasAvailableResources to be able to inspect the current status of the pool. This can be useful for short-curcuiting a request for example:

var p = Pool(1);

var pr = await p.request();

if (!p.hasAvailableResources) {
  return;
}

var pr2 = await p.request();

This PR also fixes #33

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