Update batch manager dispatch to track inflight and fix private blobs

[peterbroadhurst]

Batch manager updates

I believe I've found an issue in the way the batch manager and the batch processors interact
Because the messages stay in ready state while they are being dispatched, the readPage loop will still see all messages that are dispatched

Currently it attempts to re-dispatch them when it's told to rewinds, and relies on the processor to de-dup those messages
It all works from a consistency perspective, but with the pattern of workload from the long-run it's a little inefficient in terms of how many rewinds and re-processing we do. It seems this can reach the point the batch manager can get jammed up trying to do those re-dispatches to the processor, while the processor is busy flushing what it's got

This PR simplifies the logic significantly, by preventing duplicate dispatches in the batch manager.

The manager keeps a map of all in-flight dispatched sequences to processors, and the processors call back to the manager when they have flushed a batch (so the messages will no longer turn up in readPage queries).

Fix to private blobs

The private message batch dispatcher, was only sending the first blob in a batch. Meaning other messages with blobs in the batch would never be confirmed, because the blobs never arrived.

[codecov-commenter]

Codecov Report

Merging #633 (4062d62) into main (53ecb34) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #633   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          311       311           
  Lines        18842     18860   +18     
=========================================
+ Hits         18842     18860   +18     

Impacted Files	Coverage Δ
internal/batch/batch_manager.go	100.00% <100.00%> (ø)
internal/batch/batch_processor.go	100.00% <100.00%> (ø)
internal/config/config.go	100.00% <100.00%> (ø)
internal/privatemessaging/privatemessaging.go	100.00% <100.00%> (ø)

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[nguyer]

I'm not super familiar with this part of the code, but this looks good to me. Also one minor spelling thing in a comment, but not a big deal.