Fix[MQB]: app refcount is numVirtualStorages

[dorjesinpo]

The race:

1. Domain is being reconfigured with one less app
2. Domain saves the new config as d_config before proceeding with updating Apps (in Cluster thread)
3. PUT arrives and picks the "reduced" count but the storage for the App being removed is not removed yet. Queue inserts the PUT into the App storage that is about to be removed. (In queue thread)
4. Queue thread starts unregistering the App and as the result decrements the PUT refCount. Now, the count is one less than the number of Apps (N)
5. N - 1 Confirms arrive (where N is the number of Apps after reconfiguration).
6. refCount drops to zero because it is off by 1.
7. Queue attempts to remove the PUT but there is still Nth storage without confirm. And this asserts

this can manifest the other way - when we add new App

[pniedzielski]

Change looks good to me, testing locally while rebased on #427 to make sure it fixes the issue.

[bmq-oss-ci]

Build 277 of commit cb1a3a4 has completed with FAILURE

[bmq-oss-ci]

Build 278 of commit 616a7a0 has completed with FAILURE