Description of the feature

Current implementation of protected file is slow, especially for large files. At least the following 2 improvements can be applied.

1. Use file map to replace read/write a single node from/to disk.
2. Remove the encrypted part inside the file_node_t structure. The encrypted content can be directly retrieved from disk. The encryption/decryption functions can use the file mapped addresses instead so we can save the cache memory as well as the time used to copy it to/from disk.

Why Gramine should implement it?

Performance test on SGX SDK shows that applying these two changes can make reading 2 times fasters and writing 7 times faster.