Feature: serialize time-consuming operations

[GiantKing]

Feature Request

There are some time-consuming operations in Pegasus:
recall, balance, backup, bulk load, restore, manual compact, partition split

It will have an impact on online service while it proceeding. Especially they are running in concurrent. So we have to serialize these operations.

We can add an operation status lock for meta. Before a operation begins, acquire this lock first. And release this lock as it finish.