* (db/snapshot): add merge opration test
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

* (db/snapshot): add UpdatedOnField
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

* (db/snapshot): add Utils.h
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

* (db/snapshot): refactor Snapshot
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

* (db/snapshot): refactor snapshot 2
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

* (db/snapshot): refactor snapshot 3
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

* (db/snapshot): refactor snapshot 4
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

* (db/snapshot): refactor snapshot 5
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

* (db/snapshot): refactor snapshot 6
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>