[FGL] Solutions for StoragePolicy

StoragePolicy may be set on an ancestor path and used by some children files.

This storage policy are used by many operations, but there are two special operations:

• ChooseExcessRedundancies 
• BlockReconstructionWork

ChooseExcessRedundancies is used to process some excess redundancy replicas. BR and IBR may need to process some excess redundancy replicas.

It means that :

• BR and IBR needs to hold not only the global BM write lock but also the global FS write lock in phase 2.
• BR and IBR needs to hold not only the write lock of last iNode file but also the full directory tree locks in phase 3.

It will results in a low performance. 

In order to fix it, there are some solutions.

Solution 1: 

• Using a separate thread to process these excess redundancy blocks periodically, such as the Redundancy monitor.
• This thread can hold the full directory tree locks to process these blocks in phase 3.

Solution 2:(preferred)

• Try to set the storage policy to each iNodeFile, so that BM related operations can only involve the iNodeFile to which the processing block belongs.
• It's solution will not cost more memory.
• The storage policy can be set synchronously to all children iNodeFiles. setStoragePolicy, unsetStoragePolicy and rename require this operation.