fetch strategy:
Before:
- block fetching commands for different accounts were in the same wait
group, making them dependent on each iteration.
- transfers loading command was checking database for new unloaded
blocks on timeout and was in the same wait group with block fetching, so
it was often blocked until all block fetching commands finish for
iteration.
Now:
- block fetching commands run independently for each account
- transfers fetching command is run once on startup for unloaded blocks
from DB
- fetch history blocks commands are launched once on startup for
accounts with no full history loaded
- transfers are loaded on each iteration of block range check
without waiting for all ranges to be checked
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