Skip dumping the buffer in STARMAN_DEBUG=1

[miyagawa]

Starman debug mode prints the request header and body to the console, and enabling it on production could cause an incident where unwanted private information is logged. meanwhile, the other information displayed with the debug option could be still useful to monitor on a production environment.

This PR adds a new value of STARMAN_DEBUG=2 to enable request buffer logging, while STARMAN_DEBUG=1 will skip printing the request buffer. This is a breaking change if someone relies on the debug option printing the full request buffer to STDERR, but i would consider it's a reasonable change since it is explicitly named "debug" and we'll make sure to document it in Changes.