[SRU] configurable threshold for reaping dead connections and add a unit test

[gerald.yang]

[Impact]
add a configurable threshold for reaping dead connections and fix cephfs test case

[Test plan]
run ceph qa tests in: qa/tasks/cephfs/test_sessionmap.py
this also verifies the configurable threshold

steps to run:
1. build ceph from source folder
2. go to build folder
3. run all tests in test_sessionmap.py, e.g.
python ../qa/tasks/vstart_runner.py tasks.cephfs.test_sessionmap.TestSessionMap.test_mount_conn_close
python ../qa/tasks/vstart_runner.py tasks.cephfs.test_sessionmap.TestSessionMap.test_tell_session_drop
...

[Where problems could occur]
these patches only add a configuration and fix the test case
the only problem I can imagine is someone sets this config to a non-sense value

[Other info]
from upstream:
octopus backport tracker
https://tracker.ceph.com/issues/51199

octopus backport PR
https://github.com/ceph/ceph/pull/43310

[gerald.yang (gerald-yang-tw)]

first patch

[gerald.yang (gerald-yang-tw)]

second patch

[Ubuntu Foundations Team Bug Bot (crichton)]

The attachment "0001-msg-async-configurable-threshold-for-reaping-dead-co.patch" seems to be a patch. If it isn't, please remove the "patch" flag from the attachment, remove the "patch" tag, and if you are a member of the ~ubuntu-reviewers, unsubscribe the team.

[This is an automated message performed by a Launchpad user owned by ~brian-murray, for any issues please contact him.]

[James Page (james-page)]

This fix is included in 15.2.16 - see bug 1964802