Add client list, fix channel race, hjoin, tests

[kdedon]

Fixed a bug with hjoin, username tests to include the @, and removed the test for a number as the first character due to that being an unnecessary check after testing for the @ above.

I handled the two TODOs:
Add after the remove is handled by just writing the channel back into the global index. There is still a race condition where a user could /join during the time that the channel was deleted but while the add was still queued, so it is not completely fixed; I think it might be safest to just use a regular map and use an RWLock Mutex for any possible operation.

I added a client channel slice. I liberally used appends, so not sure how much of an optimization it actually was.

[kdedon]

These test cases tend to deadlock and need to be redesigned.

Edit: Fixed the test. The pipe acts like an unbuffered channel and blocks until the data is read. I modified the test handling to read from all the clients at once in different goroutines and then handle them sequentially once they had been collated. I also modified it to fully read everything before moving on to make it easier to debug/create tests.

[rogersm]

Even if the code is valuable, I think the your code to solve this issue is too complex. I need some time to think about it.

Could you do a PR for the testing only? This is something I'm happy to apply.

[dillera]

Where did we leave off on this? @kdedon did you intend to break this into 2 pieces? I hate to have this dangling out here for so long.

[kdedon]

This change makes it much easier to build tests since the entire server doesn't have to be constructed.