[couchbase2] Update Couchbase SDK to version 2.3.1.
[couchbase2] Update Couchbase SDK to version 2.3.1.

brianfrankcooper/YCSB,cricket007/YCSB,brianfrankcooper/YCSB,madhurihn/YCSB_ToyDB,madhurihn/YCSB_ToyDB,zyguan/ycsb,manolama/YCSB,manolama/YCSB,ChristianNavolskyi/YCSB,jaemyoun/YCSB,jaemyoun/YCSB,madhurihn/YCSB_ToyDB,ChristianNavolskyi/YCSB,leschekhomann/YCSB,cricket007/YCSB,zyguan/ycsb,zyguan/ycsb,brianfrankcooper/YCSB,brianfrankcooper/YCSB,manolama/YCSB,leschekhomann/YCSB,leschekhomann/YCSB,manolama/YCSB,jaemyoun/YCSB,zyguan/ycsb,ChristianNavolskyi/YCSB,ChristianNavolskyi/YCSB,jaemyoun/YCSB,cricket007/YCSB,madhurihn/YCSB_ToyDB,leschekhomann/YCSB,cricket007/YCSB