Handler States

To get understanding in which state the Handler currently is, it's necessary to implement OnixS::SURF::MarketData::HandlerStateListener class interface and associate an instance with the Handler using OnixS::SURF::MarketData::Handler::registerHandlerStateListener member.

Below table describes all possible states for the Handler:

State	Description
`OnixS::SURF::MarketData::HandlerState::Disconnected`	The Handler is disconnected or was not executed yet.
`OnixS::SURF::MarketData::HandlerState::Disconnecting`	The intermediate state when the Handler is not disconnected but not connected.
`OnixS::SURF::MarketData::HandlerState::Connected`	The Handler is connected.
`OnixS::SURF::MarketData::HandlerState::Connecting`	The intermediate state when the Handler is not connected but not disconnected.