Add backward-compatibility checks for store xml update
Add ReuseAddress for HTTP service

This fixes the bind exception for unit tests
Add TODO to handle RO in KeyVersionFetcher
Fix multiple broken test

1. Fix an issue in AdminClient when doing cluster.xml update and
system store client had obsolete cluster topology which will cause
InvalidMetadataException
2. Fix an issue in StreamingClientTest
Update a test to prevent NPE in test
Fix test in CachingStoreClientFactoryTest
Move KeySamplerCLI and KeyVersionFetcherCLI to tools package
fix NPE thrown on getAll requests with single key
Incorporating code review comments for changes in R2Store