Allow MineBlocksOnDemand on mocknet

[Jouzo]

What kind of PR is this?:

/kind feature

What this PR does / why we need it:

• Allows MineBlocksOnDemand

[prasannavl]

Good thought. Would it be better to set fNoRetargeting only on mocktime being set?

Alternatively, we can remove the branch that currently overrides for mocknet, and reuse mocktime to use a default proof of work limit from params.pos.diffLimit.

[Jouzo]

Good thought. Would it be better to set fNoRetargeting only on mocktime being set?

Alternatively, we can remove the branch that currently overrides for mocknet, and reuse mocktime to use a default proof of work limit from params.pos.diffLimit.

Agree. The idea behind this PR is to be able to use setmocktime in order to generatetoaddress faster, as being done in test framework.
I changed a bit the logic, reverted setting fNoRetargeting on mocknet and updated MineBlocksOnDemand directly to restrict side effects.