feat!: drop node8 support, support for async iterators

[alexander-fenster]

BREAKING CHANGE: The library now supports Node.js v10+. The last version to support Node.js v8 is tagged legacy-8 on NPM.

New feature: methods with pagination now support async iteration.

[codecov]

Codecov Report

Merging #682 into master will increase coverage by 2.80%.
The diff coverage is 99.48%.

@@            Coverage Diff             @@
##           master     #682      +/-   ##
==========================================
+ Coverage   96.20%   99.00%   +2.80%     
==========================================
  Files          15       15              
  Lines       14490    15136     +646     
  Branches      872      932      +60     
==========================================
+ Hits        13940    14986    +1046     
+ Misses        547      147     -400     
  Partials        3        3              

Impacted Files	Coverage Δ
src/index.ts	99.79% <90.90%> (-0.21%)	⬇️
src/v2/bigtable_table_admin_client.ts	98.34% <99.34%> (+6.46%)	⬆️
src/v2/bigtable_instance_admin_client.ts	98.21% <99.70%> (+7.70%)	⬆️
src/filter.ts	100.00% <100.00%> (ø)
src/instance.ts	99.82% <100.00%> (ø)
src/mutation.ts	100.00% <100.00%> (ø)
src/row.ts	100.00% <100.00%> (ø)
src/table.ts	99.84% <100.00%> (+<0.01%)	⬆️
src/v2/bigtable_client.ts	96.10% <100.00%> (+4.84%)	⬆️
... and 2 more

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