statesync: broadcast snapshot request to all peers on startup

[erikgrinaker]

On startup, the peer-to-peer stack may have peers connected before the state sync process begins, causing these to not trigger AddPeer events and thus not be used for snapshot discovery. Broadcasting a snapshot request to these explicitly makes sure we discover snapshots from existing peers as well.

[codecov]

Codecov Report

Merging #5320 into master will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #5320      +/-   ##
==========================================
+ Coverage   62.52%   62.53%   +0.01%     
==========================================
  Files         259      259              
  Lines       26766    26768       +2     
==========================================
+ Hits        16735    16739       +4     
  Misses       8574     8574              
+ Partials     1457     1455       -2     

Impacted Files	Coverage Δ
statesync/reactor.go	41.31% <0.00%> (-0.51%)	⬇️
p2p/switch.go	65.88% <0.00%> (-1.83%)	⬇️
blockchain/v2/reactor.go	35.16% <0.00%> (-1.47%)	⬇️
statesync/syncer.go	79.44% <0.00%> (-0.80%)	⬇️
consensus/reactor.go	74.68% <0.00%> (-0.69%)	⬇️
blockchain/v0/pool.go	79.29% <0.00%> (-0.32%)	⬇️
p2p/conn/connection.go	79.75% <0.00%> (-0.25%)	⬇️
consensus/state.go	71.70% <0.00%> (-0.18%)	⬇️
privval/signer_endpoint.go	81.57% <0.00%> (ø)
blockchain/v0/reactor.go	64.01% <0.00%> (+0.93%)	⬆️
... and 7 more