fix: Update SingleConnection to not hold the monitor on upcalls

[dpcollins-google]

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Fixes #<issue_number_goes_here> ☕️

[codecov]

Codecov Report

Merging #151 into master will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##             master     #151      +/-   ##
============================================
- Coverage     61.18%   61.16%   -0.02%     
  Complexity      417      417              
============================================
  Files            92       92              
  Lines          2146     2145       -1     
  Branches        180      181       +1     
============================================
- Hits           1313     1312       -1     
  Misses          736      736              
  Partials         97       97              

Impacted Files	Coverage Δ	Complexity Δ
...oud/pubsublite/internal/wire/SingleConnection.java	73.68% <66.66%> (-0.35%)	18.00 <0.00> (ø)

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