[Demo] Add session timeout in demo app

Signed-off-by: Miguel M. Almeida <miguelcma@gmail.com>
[Identifiers] Add public methods to get entity identifiers

* `[[Liquid sharedInstance] userIdentifier]`
* `[[Liquid sharedInstance] deviceIdentifier]`
* `[[Liquid sharedInstance] sessionIdentifier]`

Signed-off-by: Miguel M. Almeida <miguelcma@gmail.com>
[Fix] some sessions that weren't being closed

When `identifyUser*` methods where used during the same app launch,
previous session wasn't being closed.

Signed-off-by: Miguel M. Almeida <miguelcma@gmail.com>
[Fix] track: method that could use incorrect entity instances

`User`, `Device` and `Session` instances that were used instances may not be the correct ones in some situations.
References for the correct instances are now used on `track:` method, even if they're changed in the meanwhile.

Signed-off-by: Miguel M. Almeida <miguelcma@gmail.com>
[Reset] change the delay time that reset method waits

from 1 second to 0.2 seconds

Signed-off-by: Miguel M. Almeida <miguelcma@gmail.com>
[Demo] add button to log entity identifiers to console

* User unique_id
* Device unique_id
* Session unique_id

Signed-off-by: Miguel M. Almeida <miguelcma@gmail.com>