sync2: multipeer: fix edge cases

[ivan4th]

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Motivation

Split sync could become blocked when there were slow peers. Their
subranges are assigned to other peers, and there were bugs causing
indefinite blocking and panics in these cases. Moreover, after other
peers managed to sync the slow peers' subranges ahead of them, we need
to interrupt syncing against the slow peers as it's no longer needed.

In multipeer sync, when every peer has failed to sync, e.g. due to
temporary connection interruption, we don't need to wait for the full
sync interval, using shorter wait time between retries.

Description

This fixes aforementioned multipeer sync issues, and adds tests.
It also adds sync interval randomization to avoid network load spikes.

[codecov]

Codecov Report

Attention: Patch coverage is 71.59091% with 25 lines in your changes missing coverage. Please review.

Project coverage is 79.9%. Comparing base (9e585e5) to head (042d0d7).

Files with missing lines	Patch %	Lines
sync2/p2p.go	68.1%	13 Missing and 1 partial ⚠️
sync2/multipeer/multipeer.go	71.4%	4 Missing and 2 partials ⚠️
sync2/multipeer/split_sync.go	78.2%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##           sync2/dbset-conns   #6447     +/-   ##
===================================================
- Coverage               79.9%   79.9%   -0.1%     
===================================================
  Files                    352     352             
  Lines                  46148   46209     +61     
===================================================
+ Hits                   36894   36928     +34     
- Misses                  7162    7193     +31     
+ Partials                2092    2088      -4     

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