add option to disable syncing

[whyrusleeping]

gives flatfs a way to be less safe, but wayyyyy faster.

[rht]

As with the interface, I thought the default New constructor syncs by default? ref: #30 (comment)

[jbenet]

i'd vote for syncing by default-- but i guess this at least breaks the code and forces users to look at the new interface.

if we want to not break code, i'd suggest:

func New(path string, prefixLen int) (*Datastore, error)
func NewSync(path string, prefixLen int, sync bool) (*Datastore, error)

[rht]

As with compatibility breaking, the project that vendors and uses go-datastore directly so far is go-ipfs (that I can find from the internet). And so this can be quickly mended.

note: all the specific implementations of the Datastore interface has one version of New/NewDatastore/NewDs. Some of them use option struct (just as struct fields have default value, and hence the defaults can be set), which is more future-proof. But this is unnecessary unless there will be more options to flatfs.New in the future.