HTTP/2 connections are always active within ConnectionPool #289

>>> import httpx
>>> client = httpx.Client(http_versions=["HTTP/2"], pool_limits=httpx.PoolLimits(hard_limit=1))
>>> r = client.request("GET", "https://google.com")
19:20:50.480 - httpx.dispatch.connection_pool - acquire_connection origin=Origin(scheme='https' host='google.com' port=443)
19:20:50.480 - httpx.dispatch.connection_pool - new_connection connection=HTTPConnection(origin=Origin(scheme='https' host='google.com' port=443))
19:20:50.494 - httpx.dispatch.connection - start_connect host='google.com' port=443 timeout=TimeoutConfig(timeout=5.0)
19:20:50.559 - httpx.dispatch.connection - connected http_version='HTTP/2'
19:20:50.560 - httpx.dispatch.http2 - send_headers stream_id=1 method='GET' target='/' headers=[(b':method', b'GET'), (b':authority', b'google.com'), (b':scheme', b'https'), (b':path', b'/'), (b'user-agent', b'python-httpx/0.7.1'), (b'accept', b'*/*'), (b'accept-encoding', b'gzip, deflate, br'), (b'connection', b'keep-alive')]
19:20:50.562 - httpx.dispatch.http2 - end_stream stream_id=1
19:20:50.584 - httpx.dispatch.http2 - receive_event stream_id=0 event=<RemoteSettingsChanged changed_settings:{ChangedSetting(setting=SettingCodes.MAX_CONCURRENT_STREAMS, original_value=None, new_value=100), ChangedSetting(setting=SettingCodes.INITIAL_WINDOW_SIZE, original_value=65535, new_value=1048576), ChangedSetting(setting=SettingCodes.MAX_HEADER_LIST_SIZE, original_value=None, new_value=16384)}>
19:20:50.585 - httpx.dispatch.http2 - receive_event stream_id=0 event=<WindowUpdated stream_id:0, delta:983041>
19:20:50.585 - httpx.dispatch.http2 - receive_event stream_id=0 event=<SettingsAcknowledged changed_settings:{}>
19:20:50.618 - httpx.dispatch.http2 - receive_event stream_id=1 event=<ResponseReceived stream_id:1, headers:[(b':status', b'301'), (b'location', b'https://www.google.com/'), (b'content-type', b'text/html; charset=UTF-8'), (b'date', b'Wed, 28 Aug 2019 00:20:50 GMT'), (b'expires', b'Fri, 27 Sep 2019 00:20:50 GMT'), (b'cache-control', b'public, max-age=2592000'), (b'server', b'gws'), (b'content-length', b'220'), (b'x-xss-protection', b'0'), (b'x-frame-options', b'SAMEORIGIN'), (b'alt-svc', b'quic=":443"; ma=2592000; v="46,43,39"')]>
19:20:50.618 - httpx.dispatch.http2 - receive_event stream_id=1 event=<DataReceived stream_id:1, flow_controlled_length:220, data:3c48544d4c3e3c484541443e3c6d657461206874>
19:20:50.618 - httpx.dispatch.http2 - receive_event stream_id=1 event=<DataReceived stream_id:1, flow_controlled_length:0, data:>
19:20:50.619 - httpx.dispatch.http2 - receive_event stream_id=1 event=<StreamEnded stream_id:1>
19:20:50.619 - httpx.dispatch.http2 - receive_event stream_id=0 event=<PingReceived ping_data:0000000000000000>
19:20:50.622 - httpx.dispatch.connection_pool - release_connection connection=HTTPConnection(origin=Origin(scheme='https' host='google.com' port=443))
19:20:50.623 - httpx.dispatch.connection_pool - acquire_connection origin=Origin(scheme='https' host='www.google.com' port=443)
# ... deadlock