Enable dns cache in Connector by default, use aiodns resolver by default if available.

[asvetlov]

Casual user doesn't tune TCPConnector usually.
Thus settings should provide the best performance by default.

Taking into account that aiodns respects /etc/hosts now it's safe drop-in replacement for regular threaded resolver.

[lock]

This thread has been automatically locked since there has not been
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related bugs.

If you feel like there's important points made in this discussion,
please include those exceprts into that new issue.