Support PYCBC_LOG_LEVEL

If someone sets PYCBC_LOG_LEVEL in environment variables (and doesn't explicitly configure python logging), we should create a console logger in the cxx client and set the level appropriately. Oddly, I thought I did this, but apparently not.

Lets check for this when we create a connection object - first check to see if there's a logger already. If not, and PYCBC_LOG_LEVEL is set, make the console logger and set the level. Similarly, if we are using python logging, we need to check to see if the logger already exists and raise an exception so the user knows that the logger has to be set before ever making the first connection in cases where the PYCBC_LOG_LEVEL is set. Simple enough.