Support sending requests with reconnected client

[charbonnierg]

This pull request fixes a bug (or at least a non documented behaviour). It brings very little change to the codebase.

Problem description

• It is not possible to send requests using a client which has been connected twice.

Reproducible example:

from nats import NATS

nc = NATS()

await nc.connect()
await nc.close()
await nc.connect()
# We expect a NoRespondersError but we get a TimeoutError without this PR
await nc.request("some.unknown.subject", timeout=0.1)

Objectives of the PR

• Let users send requests with clients which were already closed before

Proposal implementation

• Reset Client._resp_sub_prefix at the end of the Client._close method.

I saw that Client._client_id is already resetted at that time, so I though that it was the right place to reset Client._resp_sub_prefix too.

How was this PR tested ?

I added a single test which is very similar to the example above. If we can talk to NATS server, then a request on a subject without subjecription must raise a NoRespondersError, if not, a TimeoutError is raised and test fails.

[charbonnierg]

When I run yapf on my branch, I get a lot of modifications:

        modified:   nats/aio/subscription.py
        modified:   nats/errors.py
        modified:   nats/js/api.py
        modified:   nats/js/client.py
        modified:   nats/js/errors.py
        modified:   nats/js/kv.py
        modified:   nats/js/manager.py
        modified:   nats/nuid.py
        modified:   nats/protocol/parser.py
        modified:   tests/test_client.py
        modified:   tests/test_client_async_await.py
        modified:   tests/test_client_nkeys.py
        modified:   tests/test_client_v2.py
        modified:   tests/test_js.py
        modified:   tests/test_nuid.py
        modified:   tests/test_parser.py
        modified:   tests/utils.py

so I did not run yapf before the commit

[wallyqs]

LGTM, some clients (like Go) do not expose connect on the instance type to avoid reusing, but in the case of Python this approach would be more convenient since we do allow creating a NATS client instance to connect besides the nats.connect method from the module.