blinding_derivation_key should be generated in HD path

[instagibbs]

Currently all keys except the blinding derivation key are generated via HD wallet. Using GenerateNewKey would enable HD derivation of that key making backups easier, and would allow us to keep it encrypted like any other key when not in use.

[dgpv]

for reference - relevant issues: #233 #448

[instagibbs]

Note that anytime the wallet is encrypted for the first time, or for example sethdseed is called(elements-0.17), the derived key will become "old" and still require a proper backup. That, or we then have to store N-many master blinding keys, and try all of them(no thanks).

One safety measure could be to print out the master blinding key directly during dumpwallet right after the xprv, and allow it to be set using sethdseed or another possibly hidden RPC call, since it can be quite dangerous to call this.