fix(core/listener): wait listener to shutdown before exit

[walldiss]

Previously we observed panic with leveldb: closed in integration tests. It was caused by core.listener accessing already closed core block generator. The reason for it could be leaked listener subscriber routine after node is closed, that was trying to access generator.

The PR introduces graceful shutdown for listener, that will wait for spawned routines to stop before exiting.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 61.53846% with 5 lines in your changes missing coverage. Please review.

Project coverage is 46.50%. Comparing base (2469e7a) to head (8df8873).
Report is 311 commits behind head on main.

Files with missing lines	Patch %	Lines
core/listener.go	61.53%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3775      +/-   ##
==========================================
+ Coverage   44.83%   46.50%   +1.67%     
==========================================
  Files         265      314      +49     
  Lines       14620    17999    +3379     
==========================================
+ Hits         6555     8371    +1816     
- Misses       7313     8609    +1296     
- Partials      752     1019     +267     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[Wondertan]

Please explain how it fixes the flake either in the comment or PR description for future ref

[renaynay]

Nit: Set chan to nil when done ? And make chan on start ?