Add semaphore to limit subchannel connect to prevent race conditions

[JamesNK]

Addresses #2420

I have a theory that concurrent calls to ConnectTransportAsync could put a subchannel in a bad state. Two connection requests happen simultaneously but a race condition occurs as they exit.

Before:

1. Connect 1 starts.
2. Connect 2 starts. Cancels connect 1.
3. Connect 2 succeeds.
4. Connect 2 exits and updates subchannel status to Ready.
5. Connect 1 exits and updates subchannel status to TransientFailure.

This PR limits access to connect with a semaphore. A subchannel will have one connect in progress at a time.

After:

1. Connect 1 acquires semaphore and starts.
2. Connect 2 tries to start. Cancels connect 1. Waits with semaphore for connect 1 to finish.
3. Connect 1 exits and updates subchannel status to TransientFailure.
4. Connect 1 releases semaphore.
5. Connect 2 acquires semaphore.
6. Connect 2 succeeds.
7. Connect 2 exits and updates subchannel status to Ready.
8. Connect 2 releases semaphore.

Note: New test is unrelated to this exact problem. I wrote it while considering another theory. It's a good test to keep around.