Parameter names in connect() are inconsistent with env var names

[maxalbert]

Recent PR's (esp. #610) have done a great job at harmonising the names of env vars and parameters. However, flowmachine.connect() still has a couple of the old names left over which are slightly inconsistent with the new env var names.

For consistency, these should be renamed as follows:

• db_port -> flowdb_port
• db_user -> flowdb_user
• db_pass -> flowdb_password
• db_host -> flowdb_host
• db_connection_pool_size -> flowdb_connection_pool_size
• db_connection_pool_overflow -> flowdb_connection_pool_overflow