* [ThreadPoolTask] Rename `done` field to `avaialable`, for better
  clarity in the contexts it's used.

* [ThreadPoolTask] Add `awaitAndAcquireFirstAvailableTask` &
  `blockUntilCompletion` functions, remove `queue` function,
  replace usages of `queue` and code which would now duplicate
  the aforementioned additions.

* [ThreadPoolTask] Add `result` field for signalling the result
  of the callback.

* [gossip] Make use of `ThreadPoolTask` instead of manually implementing
  the VerifyMessageTask Task interface details.

* [gossip] Add `GOSSIP_VERIFY_PACKET_PARALLEL_TASKS` constant, and use
  it instead of hardcoding the tasks allocated in `verifyPackets`.

* [gossip] Make `verifyPackets` and `processMessages` accept an
  a shared allocator, and make `run` provide a thread-safe allocator
  which will only be contended by a maximum of the avilable tasks
  plus the number of messages processed at the same time.

* [tar] Panic on some invalid but possible invariants.

* Constify more things that don't need to be mutable.

* Rename `SOCKET_TIMEOUT` to `SOCKET_TIMEOUT_US`, to signify its
  magnitude is in microseconds.

* Remove `recvMmsg` and simplify the timeout logic to simply setting
  it directly before the loop, and finishing the batch on the first
  `error.WouldBlock` when the batch isn't empty.

* Use `packet_batch` as an actual arraylist instead of as a weird slice.

Re-implement the logic of `run` as a function that returns the handles
which can then be joined by the caller.
Most usage sites require a bit of thought with respect to the
order of operations, so `GossipService.run` remains as a wrapper
around the new logic.
This also makes it take a `message_allocator`.