Support staking with P2SH

[Gnappuraz]

Is your feature request related to a problem? Please describe At the moment the default address retrieved by getnewaddress() is a P2SH address with segwit support, when such address is used as a recipient for a block reward for example, the scriptPubkey will be a P2SH-P2WPKH and this type of script is not stakeable.

Describe the solution you'd like As we did for #431 we can make P2SH outputs of which we know the script stakeable in IsStakeableByMe().

Describe alternatives you've considered What could also be done is returning a different address type by default, that would make also sense since we want to support segwit by default but we are not returning a native segwit address by default.