[LibOS] Add Page Cache feature for trusted files

[sahason]

check issue #1712 for details.

This PR introduces a manifest option sgx.trusted_files_cache_size which specifies the cache size allocated for trusted files. A trusted file is cached if the file is opened more than 10 times. Cache eviction policy is Least Recently Used aka LRU. This optimization is used when there are frequently-reused files.

Below are the results with and without this PR for nginx run:

Marit	Threads	Native	SGX (without page cache)	SGX (with page cache)	Degradation without page cache	Degradation with page cache
Requests/sec	64	965844	386647	759754	60%	21%

Fixes #1712

How to test this PR?

Change worker_processes value to auto in CI-Examples/nginx/nginx-gramine.conf.template
Add open_file_cache max=1024 inactive=10s; under http { block in CI-Examples/nginx/nginx-gramine.conf.template

cd gramine/CI-Examples/nginx
make clean && make SGX=1

Native run:

./install/sbin/nginx -c conf/nginx-gramine.conf
wrk -t64 -c300 -d30s http://127.0.0.1:8002/random/10K.1.html

SGX run:

gramine-sgx nginx
wrk -t64 -c300 -d30s http://127.0.0.1:8002/random/10K.1.html

Initial 1-3 run with gramine-sgx might give low performance results as trusted file chunks are added to the page cache for the first time. After 1-3 iterations results become consistent.

---

This change is