Accessing local storage outside the container when reading objects

Is your feature request related to a problem? Please describe.

currently, SN object read (GET/HEAD/RANGE) handler always accesses local storage. If the object is present, SN does not access the container members

normally, SN outside the container does not store its objects on a permanent basis. There is a transitional state when SN, after the epoch change, leaves the container and begins to migrate data to a new location. The faster this is done, the more efficiently the cluster functions

in total:

1. local lookup makes sense, and sometimes even necessary
2. hitting an object is either completely impossible or possible, but the probability drops rapidly over time

i think all this makes unconditional touching of local storage impractical

Describe the solution you'd like

manage list of to-migrate containers for which SN stores data locally but does not belong to. Iff requested container is non-local and to-migrate one, skip local read

the list is update on:

• SN init
• new epoch in runtime
• container migration finish

having this list, whole migration procedure may go faster. Afaik it is object-by-object now

Describe alternatives you've considered

no for now

Additional context

i've run 10m load to GET object of 4K payload size from REP 1 container through non-container SN. Here's cpu.pb.gz profile of proxy node. As we can see, local execution takes ~20% time

Trace the SN GET/HEAD/RANGE read handler and the lifecycle points named in the issue: SN initialization, new epochs in runtime, and migration completion. Define how the to-migrate container list is maintained and verify that reads for non-local containers skip local storage while other reads retain current behavior.