Summary:
As the C++ part exposes now SequenceNumber retrieval
for Snapshots we want this obviously also in the Java API.

Test Plan:
make rocksdbjava
make jtest
mvn -f rocksjni.pom test

Reviewers: yhchiang, adamretter, ankgup87

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D32571