JDC executes fallback system too frequently

[GitGab19]

While digging into issue #844, I discovered that JDC enters in the fallback mechanism too frequently.
I discovered the reason for that, and it's related to the prev_hash contained in shares.

• Sometime (I still don't know exactly why) a valid share is sent to the pool (in this case through JDC) with the correct prev_hash.
• When the valid share is checked by the pool, the pool uses a wrong prev_hash in the check_target function, and so the share is not a valid one for the pool.
• Pool sends a SubmitShareError message to JDC
• JDC thinks that Pool is wrongly refusing the valid share and pool-fallback is started

JDC logs:

Pool logs:

[pavlenex]

Done in #904