Partially reconstruct Active/Standby clusting.

In this mode one head is in Active state, supporting all commands, while
another is in Standby state, supporting only minimal LUN discovery subset.

It is still incomplete since Standby state requires reservation support,
which is impossible to do right without having interlink between heads.
But it allows to run some basic experiments.

(cherry picked from commit cff404ede2589a381844d8d66ed8e0e6b4a3d33b)
Log errors for absent LUNs too.

MFC after:	1 week

(cherry picked from commit 001307cceca8fa1d69b7a6e74f1d0542f157c2f2)