RegisterTLSConfig can panic with "fatal error: concurrent map writes"

[daniel-nichter]

If multiple goroutines call RegisterTLSConfig Go can panic because the internal map is not guarded: tlsConfigRegister[key] = config

Affects all versions of driver including latest SHA as of today.

Suggested fixes:

1. Guard the map, or
2. Document that this func isn't thread-safe and caller should only call it once

Best is probably option 1 and then document that callers shouldn't (and don't need to) re-register the same TLS config more than once (unless, of course, certs change).