[FGL] Solutions for Quota

Concurrent operations on directory tree may cause Quota updates and verification to not be thread-safe.

For example: 

1. Supposing there is directory /a/b and quota is on inode a and b
2. There are some directories and files under /a/b, such as: /a/b/c/d1, /a/b/d/f1.txt
3. Supposing there is a create operation under /a/b/c/d1 and there is a addBlock operation on /a/b/d/f1.txt
4. These two operations can be handled concurrently by namenode
5. They will update the quota on inode a concurrently since these operations just hold the read lock of the inode a and b.
6. so we should make quota-related thread safe.

There are two solutions to make quota-related thread safe。

Solution one: Hold the write lock of the first iNode with Quota set when resolvePath

• Directly hold the write lock of iNode a so that all operations involving subtree /a can be handled safety.
• Due to lower concurrency, maximum improvements cannot be achieved.
• But the implementation is simple and straightforward.

Solution two: Lock all QuotaFeatures during quota verification or update

• Still hold the read lock of iNode a and b
• Lock all QuotaFeatures involved in this operations, when validating or updating quotas.
• Maximum improvements can be achieved.
• But the implementation is a little complex
  • Add a lock for each QuotaFeature
  • Acquire locks for all involving QuotaFeature