Fix uniform open() func between stores

[Shane-XB-Qian]

1. uniform store.open() to/with (dir, inmemory) args
2. uniform db.open() and remove OpenWithStore()
3. modify test for that
4. modify example for that

[ostafen]

@Shane-XB-Qian, for setting engine-specific options, the user should call OpenWithStore(). There is no need to support all possible combinations on parameters in the default Open() function by adding a map.

[Shane-XB-Qian]

for setting engine-specific options, the user should call OpenWithStore()

the default db.open() hard code to bbolt only, if to change store then use OpenWithStore()?

// felt meta/plan/idx etc seems pretty good, but 'open' related is a little confused, was designed by you?