fix(sync): change default sync config to improve reliability

[teor2345]

Motivation

Zebra is a bit slow to verify orchard blocks with lots of transactions, which can cause a failure loop.

This PR should reduce the concurrency a bit, allowing it to sync more reliably.

Solution

Concurrency:

• Decrease the default lookahead limit from 2000 to 400
• Increase the block verification timeout from 6 to 10 minutes
• Decrease the default concurrent downloads config from 50 to 25

Priority:

• Try to run the spawned download task before queueing the next download

Cancellation:

• Allow verification to be cancelled if the verifier is busy

Review

This is pretty urgent, it seems to be blocking a lot of other fixes and tests.

Reviewer Checklist

• Full sync works more reliably

[codecov]

Codecov Report

Merging #4670 (1952f11) into main (82b8d7f) will decrease coverage by 0.04%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##             main    #4670      +/-   ##
==========================================
- Coverage   78.86%   78.81%   -0.05%     
==========================================
  Files         304      304              
  Lines       37504    37509       +5     
==========================================
- Hits        29578    29564      -14     
- Misses       7926     7945      +19     

[conradoplg]

Looks good