Add prometheus output

This adds a first super-basic mode for prometheus output, which is
useful when running long neobench runs and feeding the ongoing results
directly into a monitoring system.

To enable, pass --prometheus <addr>, eg --prometheus :1234, and you'll
get two useful metrics at /metrics on that url; successful transactions
and failed transactions. From those two you can deduce rates and so on.

Merge branch 'master' of github.com:jakewins/neobench

Fix old param syntax in LDBC query

Add support for literal lists

Allow string concat

Allow string concat

Add client-side parameter expansion option

Merge branch 'master' of github.com:jakewins/neobench

Break some stuff

- Moves `-w builtin:blah` and `-w myfile` to
  `-b blah` and `-f myfile`, respectively
- Adds `-S 'RETURN 1'`
- Changes backslash-prefixed metacommands to colon-prefixwq

oh this isn't vim

Add --max-connection-lifetime