Truncate (to a configurable limit) very large lock errors

[jamadden]

Fixes #511

[davisagli]

It would be nice to mention this in the docs somewhere or mention the possibility of adjusting RS_LOCK_ERROR_MAX_LEN in the log message when it is truncated. Otherwise LGTM

[jamadden]

Like all of RelStorage's tunables, it is logged on startup, including its location. That ought to be enough for the interested observer to see what's going on. As a low-level implementation detail subject to change, and one that ought to need adjusting only in very specific circumstances, I'd prefer not to document it more than it already is.

[davisagli]

@jamadden I see that it's logged at the debug level, which would not show up in my usual deployments. I can imagine that if I ran into a truncated message because of this, and I hadn't seen this PR, I would need to come look at the code to figure out how to avoid it. Anyway, I don't feel strongly about it, since usually I wouldn't need these details.