bag_to_sql: add --progress CLI option
bag_to_sql: tidy imports, persist db connection

* work without python-progressbar
* remove unused imports
* persist the dbconnection
* support multiple bag files as inputs
watch for new nodes if started after us
get topic, url and prefix from ros params
Merge branch 'strawlab-electric'
tests: move DB connection stuff to module setup/teardown
tests: can print results with env var
allow strings with null characters to be saved to postgres
adjust value so that postgres passes test (dodgy)
adjust value so that postgres passes test (dodgy)
bugfix: actually handle backreferenced arrays correctly
test prefixes (currently failing)
bugfix: prefix tests pass
enforce prefixes start and end with '/'
bugfix: ros node also uses correct prefix