op-batcher: Fix concurrent map write

[mslipper]

When the batcher enqueues multiple transactions, multiple threads use the channel's public API at once. Since these methods modify the internal pendingTransactions map, this can lead to concurrent map access errors like this one: https://app.circleci.com/pipelines/github/ethereum-optimism/optimism/75665/workflows/09c42fc5-cd49-482a-aaef-9def14c40388/jobs/3075154/tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.05%. Comparing base (4c162a2) to head (9e50101).
Report is 6 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #13527      +/-   ##
===========================================
- Coverage    47.30%   43.05%   -4.25%     
===========================================
  Files          944      776     -168     
  Lines        78914    69395    -9519     
  Branches       803        0     -803     
===========================================
- Hits         37329    29879    -7450     
+ Misses       38826    36975    -1851     
+ Partials      2759     2541     -218     

Flag	Coverage Δ
cannon-go-tests-32	?
cannon-go-tests-64	?
contracts-bedrock-tests	?

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Files with missing lines	Coverage Δ
op-batcher/batcher/channel_manager.go	85.93% <100.00%> (+0.35%)	⬆️

... and 177 files with indirect coverage changes