[NBS] Decrease downtime during releases with BS group connection warm-up

[vladstepanyuk]

During releases, there are few downtime and we want to reduce it as much as possible. Now, we establish connections lazily - only with the first message, we want to add handles to dump groups of volume channels to disk, and then, during the bootstrap warm-up, we will establish connections with these groups and then start tablets in the new nbs process.

[SvartMetal]

to dump groups of volume channels to disk

They are already being dumped as part of TabletBootInfoBackup

nbs/cloud/storage/core/libs/hive_proxy/public.h

Line 18 in 0ab2620

TString TabletBootInfoBackupFilePath;