Add/improve some tests, especially around size limits

[chkno]

Add/improve some tests, especially around message size limits.

After reading the documentation, and the tests, I wasn't clear about what happens when one tries to read a message larger than ReadLimit, so I added a test for it:

• so future users can see this at a glance by reading the tests, and
• to emphasize that this behavior, down to the exact "Message exceeded max byte size" error message text, is part of the client API: Matching this error message text is the only way for clients to know that they need to retry their read with a larger ReadLimit. I find that I need to match this error text to detect this condition in a thing I'm writing that uses commitlog, and I'd feel better if the specific error text was guarded by a test.
• to possibly start a discussion about whether clients ought to be depending on error message text like this / if there should be a better way to signal this 'you need to retry with a larger ReadLimit' condition, (or maybe even a way for a client to know how big the ReadLimit needs to be to read the next message without continually retrying with slightly higher limits). :)

[zowens]

@chkno A more specific error message does make sense to me.