Change buffer file extension

[FantasticFiasco]

Change buffer file extensions from .json to .txt.

Fixes #169

[codecov]

Codecov Report

Merging #205 (ddaa8d1) into master (8cf1a9f) will decrease coverage by 0.20%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #205      +/-   ##
==========================================
- Coverage   91.04%   90.83%   -0.21%     
==========================================
  Files          30       30              
  Lines         860      884      +24     
==========================================
+ Hits          783      803      +20     
- Misses         77       81       +4     

Impacted Files	Coverage Δ
...og.Sinks.Http/LoggerSinkConfigurationExtensions.cs	100.00% <ø> (ø)
...nks.Http/Sinks/Http/Private/IO/DirectoryService.cs	100.00% <ø> (ø)
.../Http/Private/Durable/FileSizeRolledBufferFiles.cs	100.00% <100.00%> (ø)
...p/Private/Durable/FileSizeRolledDurableHttpSink.cs	100.00% <100.00%> (ø)
...inks/Http/Private/Durable/TimeRolledBufferFiles.cs	100.00% <100.00%> (ø)
.../Http/Private/Durable/TimeRolledDurableHttpSink.cs	100.00% <100.00%> (ø)
....Http/Sinks/Http/Private/Durable/HttpLogShipper.cs	81.72% <0.00%> (-4.31%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6de79d4...ddaa8d1. Read the comment docs.