[MIR] liburcu

The coming merge of latest multipath-tools will add a dependency to liburcu.

This was added upstream by:
       commit cb98c8837693a4a4f76895ff3e8c59bc3ba844ca
       Author: Hannes Reinecke <email address hidden>
       Date: Tue Jun 14 16:36:30 2016 +0200
           multipathd: use userspace RCU to access configuration

That is a non optional change (no configure selection or runtime detection).
Patching out the locking of the project sounds like the most error prone thing I can think of, therefore I request a MIR for liburcu.

Processing Main Inclusion Requirements:
1. Availability: The package must already be in the Ubuntu universe, and must build for the architectures it is designed to work on.

Last version fine since Xenial,former verisions go back til precise (not all archs)
 liburcu1 | 0.6.7-2 | precise/universe | amd64, armhf, i386, powerpc
 liburcu1 | 0.7.12-0ubuntu2 | trusty | amd64, arm64, armhf, i386, powerpc, ppc64el
 liburcu2 | 0.8.5-1ubuntu1 | vivid | amd64, arm64, armhf, i386, powerpc, ppc64el
 liburcu4 | 0.9.1-3 | xenial/universe | amd64, arm64, armhf, i386, powerpc, ppc64el, s390x
 liburcu4 | 0.9.2-3 | yakkety | amd64, arm64, armhf, i386, powerpc, ppc64el, s390x
 liburcu4 | 0.9.2-3 | zesty | amd64, arm64, armhf, i386, powerpc, ppc64el, s390x

2. The package is useful for a large part of our user base.
=> The package is a new build dependency or dependency of a package that we already support and in wide use by enterprise Ubuntu setups (multipath-tools).
=> The package helps meet a specific Blueprint goal (The merge of a more recent multipath-version in blueprint servercloud-z-server-core).

3. Security.
=> So far the package never showed up in any of the links referred to by "Security Checks"
=> No Executables which have the suid or sgid bit set (no executables at all btw, so no /sbin, ...)
=> No services / daemons
=> No ports
=> no addon/plugin so far used directly by
* gdnsd
* glusterfs-common
* knot
* liblttng-ctl0
* liblttng-ust-ctl2
* liblttng-ust0
* liburcu-dev
* lttng-tools
* netsniff-ng
* python3-lttng
None of these is especially security-sensitive software (filters, scanners, UI skins, etc)

4. Quality assurance
=> no config needed
=> no debconf
=> no long term outstanding bugs that matter (only one for precise which nobody seemed to care so far, but that is not crit at all)
https://bugs.launchpad.net/ubuntu/+source/liburcu
https://bugs.debian.org/cgi-bin/pkgreport.cgi?dist=unstable;package=liburcu
=> No special HW needs to be used
=> Comes with a unit test that is ran on build via dh_auto_test
=> it has a watch file

5. UI standards
=> No user-facing application, so no UI/Translation needs

6. Dependencies:
=> has only one Dependency to libc6, no suggests, recommends

7. Standards compliance
=> No known FHS/Policy issues

8. Maintenance
=> Well maintained in Debian, only a sync so far.
Since needed for the process we might set Server-Dev, given what it does Foundations might be more correct but I can't directly volunteer others :-/

9. Background information:
=> The package descriptions is comprehensive and correct IMHO:
 "This data synchronization library provides read-side access which scales
 linearly with the number of cores. It does so by allowing multiples copies of
 a given data structure to live at the same time, and by monitoring the data
 structure accesses to detect grace periods after which memory reclamation is
 possible."