Fix auto create db dir

[Shane-XB-Qian]

1. adding util existing check func
2. make bbolt.Open() auto create db dir if not exist
3. adding test for that
4. modify example for it

[Shane-XB-Qian]

db.Open() said that : // Open opens a new clover database on the supplied path. If such a folder doesn't exist, it is automatically created.

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and seems badger would auto do that.

[ostafen]

@Shane-XB-Qian, maybe we can do that in the main Open() function directly, so that we don't have to implement it for each possible storage engine. What do you think?

[Shane-XB-Qian]

that we don't have to implement it for each possible storage engine

yes and no, just because it really different on stores, per a just rough code check seems badger would auto create it by itself, // and if inMemory (it supports) it actually regardingless dir, but bbolt had to do it in user side, so i thought just make sure it do have done/created it.

[riebers-m]

When will this change be merge in the upstream branch? I hit this problem yesterday and found no further information because the Open function says that it will create a new folder if it doesnt exist. Very confusing for people new to clover.