Tell verifiers when not to parallelize accounting

[garious]

Without this patch, many batches of transactions could be tossed
into a single entry, but the parallelized accountant can only
guarentee the transactions in the batch can be processed in
parallel.

This patch signals the historian to generate a new Entry after
each batch. Validators must maintain sequential consistency
across Entries.

[codecov-io]

Codecov Report

Merging #125 into master will increase coverage by 0.05%.
The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
+ Coverage   96.29%   96.34%   +0.05%     
==========================================
  Files          17       17              
  Lines        1620     1671      +51     
==========================================
+ Hits         1560     1610      +50     
- Misses         60       61       +1

Impacted Files	Coverage Δ
src/accountant_skel.rs	95.57% <100%> (+0.41%)	⬆️
src/accountant.rs	94.27% <86.2%> (-1.03%)	⬇️
src/packet.rs	95.41% <0%> (+1.25%)	⬆️

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