txSyncer: service does not start after node catching up

[algonautshant]

The txHandler and txSyncer are stopped when catching up from a catchpoint, and started again afterwards.

But they never really restart. The main loop of these services rely on a context to stop. That context is canceled when stopping. Then, when the services are started again, the contexts remain canceled, which leads for the service to immediately end.

The fix is to create a new context when starting these services.

txSync has another control mechanism to wait for before starting the loop, which should also be reset at (re)start.

This PR fixes the txSyncer part of #4775

[codecov]

Codecov Report

Merging #4768 (6e778d7) into master (6acbd18) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4768      +/-   ##
==========================================
- Coverage   54.69%   54.68%   -0.02%     
==========================================
  Files         416      416              
  Lines       53685    53684       -1     
==========================================
- Hits        29364    29358       -6     
- Misses      21894    21900       +6     
+ Partials     2427     2426       -1     

Impacted Files	Coverage Δ
catchup/service.go	68.96% <100.00%> (+0.32%)	⬆️
rpcs/txSyncer.go	73.58% <100.00%> (-0.97%)	⬇️
network/wsPeer.go	67.06% <0.00%> (-4.30%)	⬇️
agreement/cryptoVerifier.go	67.60% <0.00%> (-2.12%)	⬇️
agreement/proposalManager.go	96.07% <0.00%> (-1.97%)	⬇️
network/wsNetwork.go	66.99% <0.00%> (-0.18%)	⬇️
ledger/accountdb.go	72.66% <0.00%> (+0.15%)	⬆️
ledger/acctonline.go	78.64% <0.00%> (+0.52%)	⬆️
ledger/blockqueue.go	85.63% <0.00%> (+1.14%)	⬆️
... and 1 more

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[brianolson]

same pattern txHandler and txSyncer, eh?

[algonautshant]

The txHandler and txSync are fixed in separate PRs with separate tests to make sure each one is fixed and tested separately before putting them together.
The fix for any of txHandler and txSync can mask the other one.
The test for txHandler fix will fail with txSync alone, and the test for txSync will fail with txHandler fix alone.

[algorandskiy]

Closes #4775