Increase maximum number of read-only mmap()s used from 1000 to 4096 o…

…n 64 bit systems.

By default LevelDB will only mmap() up to 1000 ldb files for reading and then fall back
to using file desciptors.

The typical linux system has a 'vm.max_map_count = 65530', so mapping only 1000 files
seems arbitarily small. Increase this value to another arbitrarily small value, 4096.

util/env_posix.cc

@@ -585,8 +585,8 @@ static int MaxMmaps() {
   if (mmap_limit >= 0) {
     return mmap_limit;
   }
-  // Up to 1000 mmaps for 64-bit binaries; none for smaller pointer sizes.
-  mmap_limit = sizeof(void*) >= 8 ? 1000 : 0;
+  // Up to 4096 mmaps for 64-bit binaries; none for smaller pointer sizes.
+  mmap_limit = sizeof(void*) >= 8 ? 4096 : 0;
   return mmap_limit;
 }