Problems with memory mapped files

[Archie3d]

Two problems:

• in index.go the "github.com/tysontate/gommap" package does not exist and should be replaced with "github.com/tysonmote/gommap" (or alternatively - "github.com/edsrzf/mmap-go")
• The unit test segment_test.go does not close the segment before re-opening it. This leaks the mmaped file handles (at least on Windows) which results in test failure when calling s.Remove() (unable to truncate a mmaped file). The segment must be closed (s.Close()) before calling s, err = newSegment(dir, 16, c) for the test to pass.

[ryanhoangt]

Another (potential) problem I found when testing index struct using gommap is that in Close method, before truncating the file to its actual size, we must issue an i.mmap.UnsafeUnmap() call; otherwise we could get a PathError: The requested operation cannot be performed on a file with a user-mapped section open.