Updated top level README for YCSB 0.8.0 release

[build] Added Oracle JDK 8 to .travis.yml

[hbase10] Still too many threads #691.

[googlebigtable] Fix #697 by synchronizing on the static Config object

who's address does not change.

[hbase098] Add missing `writebuffersize` documentation and note to

[hbase10] Add instructions for testing against Google BigTable's HBase API.

[couchbase2] Add Support for Couchbase SDK 2.x

modify hbase-site.xml

Add documentation on how to use the hbase10 client with Google's
Bigtable.

[googlebigtable] Add a Google Bigtable binding for testing load using

the native Protobuf GRPC based API.

[elasticsearch] Fix race condition

This commit fixes a race condition that exists between the shards being
ready to serve requests and the execution of the workload.

Fix broken synchronization meant to guard against over-creation
of cluster connection instances on startup when multiple threads.

[elasticsearch] Elasticsearch improvements

This commit fixes an issue with the index creation logic when the
elasticsearch.newdb flag is set to true. Namely, when running in local
mode the index would never exist (since the node always starts with a
clean temporary directory) but with elasticsearch.newdb an attempt would
be made to delete the index which would cause the workload to fail.

This commit also does a cleanup of the index creation logic using an
explicit create index request and passing the index settings along as
part of that index creation.

Update cassandra2-cql driver to only run tests on jdk8

[cassandra] properly clear cached containers.

clear containers before we use them rather than after, so that in the case of
exceptions we can recover.

closes #127

[S3] Close opened S3Object