registrar: add use_expired_contacts config param

[smititelu]

Pre-Submission Checklist

• Commit message has the format required by CONTRIBUTING guide
• Commits are split per component (core, individual modules, libs, utils, ...)
• Each component has a single commit (if not, squash them into one commit)
• No commits to README files for modules (changes must be done to docbook files
  in doc/ subfolder, the README file is autogenerated)

Type Of Change

• Small bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds new functionality)
• Breaking change (fix or feature that would change existing functionality)

Checklist:

• PR should be backported to stable branches
• Tested changes locally
• Related to issue #XXXX (replace XXXX with an open issue number)

Description

Allow/Disallow the usage of the expired contacts.

Useful when some problems happen with new REGISTERs; allow the usage
of old REGISTERed contacts.

Default value is 0 meaning "disallow the usage of the expired contacts".
(no changes to existing behavior)

Value can be set dinamically via:
  kamcmd cfg.set_now_int registrar use_expired_contacts 1

[miconda]

Thanks!