File System API locking

[streamich]

Add locking support as per spec for the FSA emulation on top of Node fs.

• The synchronous file handle.

  Creating a FileSystemSyncAccessHandle takes an exclusive lock on the file associated with the file handle. This prevents the creation of further FileSystemSyncAccessHandles or FileSystemWritableFileStreams for the file until the existing access handle is closed.

• Throw NoModificationAllowedError error when async write cannot succeed because of locking.