Make JDC fallback simpler

[Shourya742]

refer: #180 (comment)

Currently, JDC starts in Solo Mining mode and then transitions to FullTemplate or CoinbaseOnly mode based on the upstream connection. During fallback periods, JDC operates in Solo Mining mode until an upstream is connected. We can simplify this by avoiding multiple transitions: instead of using Solo Mining as an intermediary step, we can transition directly from the configured mode (FullTemplate or CoinbaseOnly) to Solo Mining.

[GitGab19]

Let's expand the description and revisit the title eventually here.