Add madvise setting for sequential dbs

[glinton]

Madvise allows advising the kernel how the mmap should be accessed. The default is "random", but if the db is being used sequentially, setting to "normal" has been shown to improve performance on high latency filesystem storage.

Resolves #691

Example:

madvise := syscall.MADV_NORMAL
db, err := bolt.Open(path, 0644, &bolt.Options{
	Timeout:    0,
	NoGrowSync: false,
	Madvise: &madvise,
})

The reason for the pointer is because an empty value would be "normal" (0x0), use options only if set, and keep using "random" by default.