Support asynchronous migrations

[vitusortner]

Closes #273

[codecov]

Codecov Report

Merging #287 into develop will not change coverage by %.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #287   +/-   ##
========================================
  Coverage    81.38%   81.38%           
========================================
  Files           57       57           
  Lines         1477     1477           
========================================
  Hits          1202     1202           
  Misses         275      275           

Flag	Coverage Δ
#floor	81.38% <100.00%> (ø)
#floor_generator	80.77% <ø> (ø)

Impacted Files	Coverage Δ
floor/lib/src/migration.dart	18.75% <ø> (ø)
floor_generator/lib/writer/database_writer.dart	80.55% <ø> (ø)
floor/lib/src/adapter/migration_adapter.dart	100.00% <100.00%> (ø)

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