Add store package

[NicolasMahe]

Add a new store package in database package to create a common implementation of both cosmos kvstore and goleveldb.

---

TODO:

• unit test: Write unit test of package database/store #1253

[krhubert]

no panic handle, for example has method might look like:

func (s *CosmosStore) Has(key []byte) (has bool, err error) {
  defer func() {
    if r := recover(); r != nil {
      var ok bool
      if err, ok = r.(error); !ok {
        err = fmt.Errorf("store: %s", r)
      }
    }
  }()
  has = s.store.Has(key)
  return
}

I know it's ugly, but cosmos store uses panics and there is no other option to handle this ...

[krhubert]

Also do we need store dir under database? I think we can just have database/ or store/, what do you think?

[NicolasMahe]

no panic handle, for example has method might look like:

func (s *CosmosStore) Has(key []byte) (has bool, err error) {
  defer func() {
    if r := recover(); r != nil {
      var ok bool
      if err, ok = r.(error); !ok {
        err = fmt.Errorf("store: %s", r)
      }
    }
  }()
  has = s.store.Has(key)
  return
}

I know it's ugly, but cosmos store uses panics and there is no other option to handle this ...

Could you create a PR that add the error management?

Also do we need store dir under database? I think we can just have database/ or store/, what do you think?

The files were originally in /store but i moved them in database at it is database related and only use by database. I'm open to move it back. @antho1404 what do you think?

[krhubert]

The files were originally in /store but i moved them in database at it is database related and only use by database. I'm open to move it back. @antho1404 what do you think?

Ok, could be database/store then :)

[mesg-bot]

This pull request has been mentioned on MESG Community. There might be relevant details there:

https://forum.mesg.com/t/network-implementation-with-cosmossdk/304/15

[antho1404]

Please fix the comments issue on codacy before merge