6.20.3 plus option to disable .sst preloading

This adds enum option file_preload. The 3rd setting kFilePreloadDisable allows Stardog to successfully execute DB::Open() even if some .sst files are corrupt.

Facebook 6.20.3 plus Stardog patches

Rebased stardog master to facebook master. Rebased stardog/develop to facebook's v6.20.3. Verified the rebase kept our pending patches. Tagged our patches as v6.20.3a

Fix clock disable in merge_helper and check tests of version_builder.cc

Two old items returned:

• our fix to 5.x merge_helper got lost in update to 6.x. Clock is once again disabled that causes noticeable performance loss
• fix to version_builder works fine in production but caused segfaults in check tests. Adjusted.

Remove table cache pinning

Facebook added code in release 6.0.0 that allows 25% of the table cache to have files permanently pinned. If our memory management reduces the size of the table cache, there is no space for new files. Performance tanks.

v6.16.4 adding second patch

Added patch for GetFreeSpace() to use appropriate value for non-root users.

Upsteam v6.16.4 + patches

Create DB properties for internal table cache

Add table reader GetIntProperties.

Backporting the new table reader properties from rocksdb 6.x. Also update write_buffer_manager.h with final code that was accepted by Facebook to rocksdb 6.x (Facebook found a minor flaw which is now fixed).

Upstream v6.15.4 + patches

Release https://github.com/facebook/rocksdb/releases/tag/v6.15.4 including some of our patches

Really fix Env_ this time

stardog-develop-1.13 was supposed to fix the Env_ warning messages ... but a check-in was missed. This fixes.

Correct Env_ warning

1.12 created a warning message that is just annoying during builds. This fixes it and corrects MergeHelper::FilterMerge() to match PR Facebook accepted.