Get rid of the following warning triggered by older versions of fs-extra
on Node.js 10.1.0:

  ExperimentalWarning: The fs.promises API is experimental

Pick up the new strong-docs version that does not trigger
fs.promises ExperimentalWarning on Node.js 10.1.0.

When running on Node.js 10.1.0, fs-extra triggers the following warning:

  ExperimentalWarning: The fs.promises API is experimental

Unfortunately it's not enough to upgrade to the latest fs-extra,
because many of our (deep) dependencies are still using an older
fs-extra version.

Fix DefaultCrudRepository to re-use legacy juggler Models across
all instances of the same Repository class.

Before this change, each call of DefaultCrudRepository constructor
was redefining the backing persisted model.

This commit adds a caching mechanism to DefaultCrudRepository:
when setting up a backing model, we check datasource's modelBuilder
registry to find if the backing model was not already created by
an older instance of the repository.

 - @loopback/authentication@0.10.2
 - @loopback/boot@0.10.2
 - @loopback/build@0.6.4
 - @loopback/cli@0.11.4
 - @loopback/context@0.11.0
 - @loopback/core@0.8.2
 - @loopback/metadata@0.8.2
 - @loopback/openapi-spec-builder@0.7.2
 - @loopback/openapi-v3-types@0.7.2
 - @loopback/openapi-v3@0.10.2
 - @loopback/repository-json-schema@0.9.2
 - @loopback/repository@0.10.2
 - @loopback/rest@0.10.2
 - @loopback/service-proxy@0.5.3
 - @loopback/testlab@0.10.2
 - @loopback/example-hello-world@0.7.2
 - @loopback/example-log-extension@0.9.2
 - @loopback/example-rpc-server@0.7.2
 - @loopback/example-todo@0.9.2
 - @loopback/docs@0.7.4