[SQLite] Set the synchronous flag to OFF

Summary:
On spinning disks the performance of commits are worse when `synchronous=NORMAL`.
Reading the documentation of SQLite, when `synchronous=OFF` there's a risk of DB corruption when the operating system crashes or the computer loses power before that data has been written to the disk surface; on the other hand, a crash in Infer should keep data in the DB in a sound state.
Buck reached the same conclusions too: 4680162279

Reviewed By: mbouaziz

Differential Revision: D6413384

fbshipit-source-id: 99e4650

infer/src/base/ResultsDatabase.ml

@@ -114,7 +114,7 @@ let new_database_connection () =
       database_fullpath
   in
   Sqlite3.busy_timeout db 10_000 ;
-  SqliteUtils.exec db ~log:"synchronous=NORMAL" ~stmt:"PRAGMA synchronous=NORMAL" ;
+  SqliteUtils.exec db ~log:"synchronous=OFF" ~stmt:"PRAGMA synchronous=OFF" ;
   database := Some db ;
   List.iter ~f:(fun callback -> callback db) !new_db_callbacks