Add a soft limit for mailbox and service queues (v2)

[michal42]

Add a size-warn limit to mailbox and service configuration, to log when a queue grows too much, without dropping messages.

Most of this PR is cleanups and tests, the logging is added by the last two commits.

Changes since https://github.com/zeromq/malamute/pull/298:

• Avoid c99-ism mlm_mailbox_bounded_test
• Use a simpler way of passing configuration to the mailbox actor
• Regenerate with current zproject to make travis happy