Disable retargeting for regtest - option 2

[oscarguindzberg]

Retargeting is disabled on bitcoin core for regtest, but enabled on bitcoinj for regtest.

See https://github.com/bitcoin/bitcoin/blob/master/src/chainparams.cpp#L257
See https://github.com/bitcoin/bitcoin/blob/master/src/pow.cpp#L54

If you have a bitcoind running on regtest with more than 10k blocks, and use bitcoinj to sync with it, it will reject blocks starting on block 10,000.

I wrote 2 different solutions:

Solution 1: The way it is implemented on bitcoin core (The other PR I created #1273)
Solution 2: Simple solution (this PR)

[coveralls]

Coverage increased (+0.06%) to 71.707% when pulling 3d322b7 on oscarguindzberg:noretargeting2 into f416dec on bitcoinj:master.

[coveralls]

Coverage remained the same at 71.65% when pulling 045a5e9 on oscarguindzberg:noretargeting2 into 8bd47a6 on bitcoinj:master.

[schildbach]

Merged.