allow multiple obfuscation transports per host

If a provider's gateway has multiple obfuscation transports per gateway configured (e.g. obfs4 and obfs4+kcp), Bitmask is not able to choose either of them. Instead it always uses the last configured obfs4 protocol of a gateway. The reason for it is that internally, the app stores transports in a hash map with the transport type as a unique key, which doesn't work if obfs4 occurs twice. We need to change the way we hold and handle the transport data internally.

as a result of testing 1.4.0 RC2 against the demo provider.