Making LookupClient disposable and improved TCP client pool

[MichaCo]

Making LookupClient and DnsMessageHandlers disposable.
Primarily to dispose the TcpClient-pool.
The pool could be empty if the TCP fallback was either disabled or never used, so it is not always necessary to dispose the LookupClient owning that resource.

[antonfirsov]

A couple of remarks. Since this is a significant breaking change, I would recommend to signal it to the package consumers by a major version bump.

[antonfirsov]

The caller Query method doesn't seem to translate cancellations to TimeoutException.

[MichaCo]

Not exactly sure what you mean by that.
What I want to do here is allowing LookupClient does handle OperationCanceled- and Timeout-Exceptions and retry the request if possible.
For TCP, if the connection attempt takes longer then the configured timeout for a query, I want to handle it the same way and maybe have LookupClient retry.

[MichaCo]

changed both, sync and async version to throw new OperationCanceledException("Connection timed out.", cancellationToken);
Still, its mostly a signal for LookupClient to retry

[antonfirsov]

Since this method is async, it shouldn't be translated to TimeoutException.

[MichaCo]

I'll change both, the sync and async version to throw new OperationCanceledException("Connection timed out.", cancellationToken);