kms: add support for stack trace levels (#20)

This commit adds support for requesting stack traces
only for specific log records.
Now a client can request stack traces only for records
with a certain log level. This allows a client to filter
out non-interesting stack traces (e.g. for debug log records).

While clients can always filter themselves, not sending
them at the server-side saves bandwidth and CPU time used
when marshaling.

Signed-off-by: Andreas Auernhammer <[email protected]>