Abilty to support upto a million .sst files in the database

Summary:
There was an artifical limit of 50K files per database. This is
insifficient if the database is 1 TB in size and each file is 2 MB.

Test Plan: make check

Reviewers: sheki, emayanke

Reviewed By: emayanke

CC: leveldb

Differential Revision: https://reviews.facebook.net/D8919

db/db_impl.cc

@@ -118,7 +118,7 @@ Options SanitizeOptions(const std::string& dbname,
   Options result = src;
   result.comparator = icmp;
   result.filter_policy = (src.filter_policy != nullptr) ? ipolicy : nullptr;
-  ClipToRange(&result.max_open_files,            20,     50000);
+  ClipToRange(&result.max_open_files,            20,     1000000);
   ClipToRange(&result.write_buffer_size,         64<<10, 1<<30);
   ClipToRange(&result.block_size,                1<<10,  4<<20);
   if (result.info_log == nullptr) {