Asyncio Client improvements

Nobody has claimed this yet.

After thoroughly examining the module, I found multiple improvements could/should be done to asynchronous code. I'd like to open a discussion before actually implementing the code because **some changes will break the current API**.

- [ ] (**breaks API**) `AioSimpleIRCClient` currently cannot be created and started once event loop has started and running asynchronous code, because `AioConnection.connect` calls `loop.run_until_complete`. In the situation described above, a `RuntimeError: This event loop is already running` exception is raised. `AioConnection.connect` should be a coroutine. Waiting for it's finishing should be caller's responsibility. Changing this function to a coroutine definitely breaks the current API.
- [ ] Connection reset by peer should be handled.
- [x] SSL should be supported. Current async code does not support ssl. `connection.Factory` should be adapted to async code as well.
- [ ] Circular reference between `AioConnection` and its underlying `protocol` should be removed.
- [ ] Async version of `bot` should be added.
- [ ] Debug usage of `print` should be removed. `print` is used 3 times in `client_aio.py`. I assume the developer forgot to remove them.
- [ ] Make the use of custom event loops easier. If one wants to use `AioSimpleIRCClient` with a custom event loop, subclassing both `AioReactor` and `AioSimpleIRCClient` is required. It is supposed to be as simple as calling `AioSimpleIRCClient(loop=loop)`
- [ ] `loop` should not be accepted as a positional argument. For example, `AioReactor.__init__` should be declared as

> `def __init__(self, on_connect=__do_nothing, on_disconnect=__do_nothing, *, loop=None)`

- [ ] `asyncio.Lock` should be used over `threading.Rlock`.
- [ ] Async throttler
- [ ] Required version check
- [ ] Better tests.

Start with client_aio.py and the AioConnection.connect and AioReactor entry points named in the issue, then review the checklist and existing asynchronous tests. The work is complete only after the selected API, connection-handling, event-loop, locking, throttling, and testing improvements are agreed and implemented without leaving the listed issues unresolved.

Written by the indexing model from the issue text.