perf: handle sync and engine messages in same loop

[mattsse]

this improves the engine poll loop hinted at in #3179

this should reduce the time we're potentially unresponsive to engine messages because we no longer drain all download events at once and instead alternate between both sources like: select! { engine messages from rpc, sync event }

this way we only process 1 download response before checking engine messages again

[onbjerg]

lgtm overall, although I'm not too knowledgeable about this part of the code, so will defer to @rkrasiuk or others

[codecov]

Codecov Report

Merging #3276 (7ca1da6) into main (8eeba4d) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3276      +/-   ##
==========================================
- Coverage   69.72%   69.71%   -0.01%     
==========================================
  Files         530      530              
  Lines       71531    71529       -2     
==========================================
- Hits        49876    49868       -8     
- Misses      21655    21661       +6     

Flag	Coverage Δ
integration-tests	16.37% <0.00%> (-0.01%)	⬇️
unit-tests	64.76% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
crates/consensus/beacon/src/engine/sync.rs	74.50% <ø> (-1.97%)	⬇️
crates/consensus/beacon/src/engine/mod.rs	78.60% <100.00%> (+0.11%)	⬆️

... and 6 files with indirect coverage changes

[Rjected]

Nice, yeah I see how the alternating loop acts like a select, returning if both are Pending