add failing testPushRetry() which exposes issue

https://github.com/couchbase/couchbase-lite-android/issues/398
Use threadsafe data structures

https://github.com/couchbase/couchbase-lite-android/issues/398
Add RemoteRequestTest.testRetry() - passes

https://github.com/couchbase/couchbase-lite-android/issues/398
Make RemoteRequestTest.testRetry() more realistic, now it fails

https://github.com/couchbase/couchbase-lite-android/issues/398
Add test for RemoteRequest retry logic

https://github.com/couchbase/couchbase-lite-android/issues/398
testPushRetry() demonstrates issues with Push replicator retry logic.

https://github.com/couchbase/couchbase-lite-android/issues/398