Improvements to `log` sub command

It would be good if `composer server log` allowed some more options:

- Ability to clear the logs for a given service
- Ability to pass `-n [number]` to only output the last X lines of the log
- Displaying logs for a service need not show the service name on every line

Start by locating the implementation and tests for the `composer server log` command. Check how service logs are currently selected and displayed, then assess the three requested behaviors: clearing a service's logs, limiting output with `-n`, and omitting repeated service names; the work is done when each behavior is covered and verified.

Written by the indexing model from the issue text.