Migrate codeebase to pure kotlin

[Kaaveh]

There are three .java files and I converted them to Kotlin:

1. This file: core/src/main/java/com/farsitel/bazaar/util/AbortableCountDownLatch.java
2. There are two functional interfaces that can not convert to Kotlin. First, I upgrade Kotlin version to 1.4.0 to use SAM conversions, then migrate BazaarStorageCallback and BazaarSignInCallback to Kotlin.