[release/1.1] Fix context cancelled error after client close

[austinvazquez]

Issue

#137

Description

Closes the receiver goroutine without setting the client error when the client is closed. Client error will be set on next dispatch call invoked if any.

Testing

Adds unit test which calls test service call after client is closed. Before the change, the test failed 1 in every ~3000 runs. After the change, the test passed successfully 10000 runs in a row.

Run test:

go test -timeout 300s -tags unit,integration -run ^TestClientReturnsErrClosedAfterClose$ github.com/containerd/ttrpc -v -race -count=10000

Test failure without fix:

=== RUN   TestClientReturnsErrClosedAfterClose
    client_test.go:100: Expected ErrClosed after connection has been closed, got context canceled
--- FAIL: TestClientReturnsErrClosedAfterClose (0.00s)
FAIL
FAIL    github.com/containerd/ttrpc     0.648s
FAIL

Output:

=== RUN   TestClientReturnsErrClosedAfterClose
--- PASS: TestClientReturnsErrClosedAfterClose (0.00s)
=== RUN   TestClientReturnsErrClosedAfterClose
--- PASS: TestClientReturnsErrClosedAfterClose (0.00s)
=== RUN   TestClientReturnsErrClosedAfterClose
--- PASS: TestClientReturnsErrClosedAfterClose (0.00s)
PASS
ok      github.com/containerd/ttrpc     7.583s

Alternative solutions considered

Considered setting the client error to ErrClosed when the receive goroutine exits on context cancel. I decided against this as closing the client is not an error until a call is made on a closed client which would result in the error being set to ErrClosed.