abci/client/local_client: do not hold mtx during callback

[melekes]

#2748 (comment)

Refs #5411

[codecov]

Codecov Report

Merging #6170 (def79b2) into master (c1ca749) will decrease coverage by 0.06%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #6170      +/-   ##
==========================================
- Coverage   60.70%   60.64%   -0.07%     
==========================================
  Files         276      276              
  Lines       25706    25707       +1     
==========================================
- Hits        15606    15591      -15     
- Misses       8478     8490      +12     
- Partials     1622     1626       +4     

Impacted Files	Coverage Δ
abci/client/local_client.go	0.00% <0.00%> (ø)
privval/signer_listener_endpoint.go	80.00% <0.00%> (-7.06%)	⬇️
privval/socket_listeners.go	78.72% <0.00%> (-4.26%)	⬇️
privval/secret_connection.go	72.68% <0.00%> (-3.61%)	⬇️
blockchain/v0/pool.go	73.76% <0.00%> (-1.91%)	⬇️
statesync/snapshots.go	91.59% <0.00%> (-1.69%)	⬇️
statesync/syncer.go	79.60% <0.00%> (-0.81%)	⬇️
consensus/reactor.go	69.74% <0.00%> (-0.79%)	⬇️
consensus/peer_state.go	84.21% <0.00%> (ø)
proxy/multi_app_conn.go	48.05% <0.00%> (ø)
... and 6 more