Aioca disconnects when repeatedly run under pytest

When repeatedly running tests against a running IOC, `aioca` will occasionally report a timeout exception when trying to establish the connection, and subsequently fail the test.

The code below is a minimal recreate of the issue - it starts a `softioc` IOC before the first test, runs 1000 attempts to get a PV, and then stops the IOC. Between one and three test failures are reported on my machine (although occasionally it reports 0).

```
import multiprocessing
import time
import pytest

def simple_ioc():
from softioc import softioc, builder, asyncio_dispatcher
dispatcher = asyncio_dispatcher.AsyncioDispatcher()
builder.SetDeviceName("ABC")
builder.longIn("PV", initial_value=0)
builder.LoadDatabase()
softioc.iocInit(dispatcher)
while(True):
time.sleep(0.1)

@pytest.fixture(scope="module")
def ioc_inline():
p = multiprocessing.get_context("forkserver").Process(target=simple_ioc)
p.start()
yield
p.kill()
p.join()

@pytest.mark.parametrize("abc", [i for i in range(1000)])
@pytest.mark.asyncio
async def test_get_pv(abc, ioc_inline):
from aioca import caget, purge_channel_caches
val = await caget("ABC:PV")
assert val == 0
```

This was tested in a new `pipenv` environment with just `aioca==1.5`, `softioc==4.2.0`, `pytest`, and `pytest-asyncio`. Tests were executed with `pipenv run pytest --tb=native -vv test.py`.

The issue is exposed due to the use of event loops in pytest. By default a new loop is created for every test. The code below overrides this behaviour, and when using it shows zero errors (it also runs significantly faster):

```
@pytest.fixture(scope="session")
def event_loop():
import asyncio
try:
loop = asyncio.get_running_loop()
except RuntimeError:
loop = asyncio.new_event_loop()
yield loop
loop.close()
```

Originally discovered by @rjwills28 while working on Coniql.