Fix object and goroutine leaks

Make sure to close resources like Buckets and ChannelMappers that hang
around till explicitly closed.

This probably isn't too significant in ordinary use, but this does
prevent hundreds of JSServer goroutines from building up during unit
testing, which makes it hard to read the stack dump if something panics.
Log warnings for invalid params to channel(), access(), etc.

…but not for legal null/undefined params.