Add overall timeout support

[karpetrosyan]

Todo

• [x] Update documentation

Hi! It seems we have received many requests for adding total timeout support.

In each place where we are using the operation (read, write, pool, connect) timeout, we should take the minimum value of the operation timeout (timeouts['pool'], for example) and the total timeout value (timeouts['total']). We should also measure the time of each operation and adjust the value of the total timeout accordingly, so we always have the correct total timeout value within the scope of the request.

I'v used the context manager syntax to simplify time measuring, also have added a simple api level test for total timeout, as we don't have any tests for the concrete timeouts