Rename or eliminate non-errroneous ssl error metrics

[zwoop]

Talking to Susan, these would be candidates for renaming, or elimination:

proxy.process.ssl.ssl_error_want_write
proxy.process.ssl.ssl_error_want_read
proxy.process.ssl.ssl_error_zero_return

The problem I have with these is that they are normal behavior, so at a minimum putting them into the error category is confusing / misleading. They might not even be useful at all, but I'll leave that to the Good Dr. to decide.

[shinrich]

Will put up a PR next week.

[shinrich]

Merged in relevant PR.