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

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

* (db/snapshot): remove one of Operations constructor
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

* (db/snapshot): add DropPartition API and related test
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

* (db/snapshot): fix bugs for multi threads
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

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

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

* (db/snapshot): some bug fix 1
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

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

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

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

(db/snapshot): update ut
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

(db/snapshot): update ut
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

(db/snapshot): update ut
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

* (db/snapshot): fix bug in ReferenceProxy
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

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