fix(GODT-1757): Refactor code to have better data separation

Due to Go's lackluster import system we had to refactor certain types
into separate packages so we can better control what each type has
access to.

All the database related code has been moved into the db package.

All the remote user management code has been been moved into the remote
package. Existing remote code has been erased.

All things related to state and snapshot have been moved into the state
package.

To avoid cyclic imports, the state package declares two interfaces which
provide access to some user specific data as well as a custom connector
implementation on which metadata can be attached.

builder.go

@@ -9,7 +9,7 @@ import (
 
 	"github.com/ProtonMail/gluon/events"
 	"github.com/ProtonMail/gluon/internal"
-	"github.com/ProtonMail/gluon/internal/backend"
+	"github.com/ProtonMail/gluon/internal/remote"
 	"github.com/ProtonMail/gluon/internal/session"
 	"github.com/ProtonMail/gluon/profiling"
 	"github.com/ProtonMail/gluon/store"
@@ -48,7 +48,7 @@ func (builder *serverBuilder) build() (*Server, error) {
 		return nil, err
 	}
 
-	backend, err := backend.New(filepath.Join(builder.dir, "backend"), builder.storeBuilder, builder.delim)
+	backend, err := remote.NewRemote(filepath.Join(builder.dir, "backend"), builder.storeBuilder, builder.delim)
 	if err != nil {
 		return nil, err
 	}