storage: Use 1M buffers for Tar transfers

[magik6k]

Should make cross-worker transfers use slightly less CPU (and potentially be faster on 10G+ networks)

[codecov]

Codecov Report

Merging #7681 (8454abc) into master (3714177) will decrease coverage by 0.06%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #7681      +/-   ##
==========================================
- Coverage   39.60%   39.54%   -0.07%     
==========================================
  Files         635      635              
  Lines       67581    67571      -10     
==========================================
- Hits        26767    26719      -48     
- Misses      36217    36243      +26     
- Partials     4597     4609      +12     

Impacted Files	Coverage Δ
extern/sector-storage/stores/http_handler.go	75.28% <33.33%> (+1.69%)	⬆️
extern/sector-storage/tarutil/systar.go	42.10% <50.00%> (-7.90%)	⬇️
extern/sector-storage/stores/remote.go	58.24% <100.00%> (ø)
chain/events/message_cache.go	87.50% <0.00%> (-12.50%)	⬇️
chain/actors/builtin/miner/diff.go	48.52% <0.00%> (-10.30%)	⬇️
chain/stmgr/execute.go	86.95% <0.00%> (-4.35%)	⬇️
storage/wdpost_sched.go	75.49% <0.00%> (-3.93%)	⬇️
chain/stmgr/call.go	71.51% <0.00%> (-3.64%)	⬇️
chain/types/tipset_key.go	85.96% <0.00%> (-3.51%)	⬇️
node/hello/hello.go	63.63% <0.00%> (-3.41%)	⬇️
... and 24 more

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