* Add DeleteGroups function to Client

* goimports

* set minimum Kafka version

* goimports

* improvements

Profiling shows that up to 40% of our allocations happen in
r.markRead(): just to allocate a slice to pass the "remain" arg, which
will be discarded because we don't run with r.debug=true.

This makes the code slightly verbose, having to check the debug mode
before invoking r.log(), but it's a huge difference in high-throughput
applications.

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

xdg/scram was moved to xdg-go/scram, the original no longer gets updated

…1103)

Add explicit errors as arguments so anyone can handle them in the logger implementation.