Prefer async methods on connection numbers to their property analogues in the Pool class

[stankudrow]

Currently the Pool class has the following properties:

1. maxsize - the maximum number of connections a pool object can spawn and no more
2. minsize - the minimum number of connections a pool object can keep opened and no less
3. acquired_connections - the number of connections requested from a pool object
4. free_connections - the number of connections that can be requested from the pool ("of sorts")

The first two are constants and can stay as properties, but the last two guys are dynamic in nature, so they are subjected to possible data races. To ensure the coroutine safety, these properties are better to get deprecated in favour of analogous get_ async methods.

For instance:

class Pool(...):
    """Pool docs"""

    @property
    def maxsize(self) -> int:
        return self._maxsize  # won't change

    @property
    def minsize(self) -> int:
        return self._minsize  # a constant

    # What a shorter variant could replace this method name?
    async def get_acquired_connections(self):
        async with self._lock:  # already defined in the `Pool` class
            return len(self._acquired_connections)  # it depends

    async def get_free_connections(self):
        async with self._lock:  # already defined in the `Pool` class
            return len(self._free_connections)  # it is dynamic

It may seem strange at the first glance, but the benefits of coroutine-safe methods are essential. So, the acquired_connections and free_connections can be deprecated in the async v0.2.6 in order to be removed in the v0.2.7.

@long2ice , what do you think?

[stankudrow]

Inspired by the PR #122 which is closed.