Producer double confirm bug after changing sign key that results in producing blocks by different nodeos

[taokayan]

similar to #3835, Under following scenario there would be double confirmation bug:

• node x is producing under account a1 using signing key k1
• a client change a1's singing key from k1 to k2
• new schedule takes place after a while
• node y receive block that produced by a1 and signed by k1
• node y start to produce under account a1 using signing key k2, but in node y's point of view, the watermark of a1 is the first time a1 was selected, resulting in double confirm bug.

proposed fix:

• reset water mark for account x if we don't have the old sign key of x (which means we were not producing for account x) upon new schedule
• also check with last produced block number in head_block_state, as we may receive a block with same producer but old signing key.