Replaced custom caching of the downloaded contracts with Gradle's
incremental build. After that `InitContractsTask` is not needed any
more, as its main purpose was to be a bridge between custom caching
and Gradle's one. Simplified code to download contracts is now part of
the `ContractsCopyTask`.
Also fixed `PublishStubsToScmTask` and removed incremental stuff from
there for now.
Fixes gh-1133
d4740706f0