Optional async

[jorgecarleitao]

This PR makes async optional, to reduce the dependency tree.

Users must how pass the feature async to activate all async functionality.

[codecov-commenter]

Codecov Report

Merging #174 (82fd4c4) into main (e556d78) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #174   +/-   ##
=======================================
  Coverage   85.49%   85.49%           
=======================================
  Files          78       78           
  Lines        7898     7898           
=======================================
  Hits         6752     6752           
  Misses       1146     1146           

Impacted Files	Coverage Δ
src/read/mod.rs	91.72% <ø> (ø)
src/write/column_chunk.rs	89.69% <ø> (ø)
src/write/indexes/write.rs	100.00% <ø> (ø)
src/write/mod.rs	75.00% <ø> (ø)
src/write/page.rs	80.00% <ø> (ø)
src/write/row_group.rs	93.37% <ø> (ø)

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