don't use master connection for on-run-start|end

[drewbanin]

This can lead to timeout errors on Snowflake and Redshift, eg:

DatabaseException: Database Error
  server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.

The master connection is used throughout the run, so it's a little perplexing that it somehow gets disconnected. In particular, it looks like this happens when dbt calls

adapter.clear_transaction(profile)

in the ModelRunner. The line that fails is a BEGIN statement.

[cmcarthur]

fixed by #693