Implement BIP39 (Mnemonic code for generating deterministic keys)

[levonpetrosyan93]

Fixes #732
Implementation of #559

https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki

High level summary:

• 12 and 24 words supported

• passphrase supported but optional.

• By default, new wallets will be created with mnemonic. Users can choose to disable a mnemonic wallet with usemnemonic=0.

• In zcoind a 24 word mnemonic with no passphrase is created by default. This is also the default setting at the Qt screen

• Qt users are shown the mnemonic page on startup, if:

  • new wallet being created
  • usemnemonic=1 (default. meaning the page is not shown if usemnemonic=0 passed)

• importwallet and importprivkey block with a mnemonic enabled wallet. A suggestion is given to create a non-mnemonic wallet if import is need. The purpose of this is to have a single mnemonic backup that is not complicated by other imports. (The keys would still import into the wallet if this check was removed, but presumably whoever was to do this would be aware of the repercussions).

• conf setting interactions are handled

• This PR also removes the code that creates a new Master key following encryption, either for a mnemonic wallet or older BIP32/44 wallets. This feature was introduced by us when Zcoin launched (ie. it was not a feature of Bitcoin 0.13 or any other version ); it provides no extra security benefit and only complicates the wallet code. Warning messages have been updated to reflect this change.

[qashban4]

@levonpetrosyan93 changes delivered as per Peter request?
@ultimaweapon can you review

[lgtm-com]

This pull request introduces 3 alerts when merging 214a027 into beb6440 - view on LGTM.com

new alerts:

• 1 for Commented-out code
• 1 for For loop variable changed in body
• 1 for Sign check of bitwise operation

[lgtm-com]

This pull request introduces 3 alerts when merging de82923 into 955e971 - view on LGTM.com

new alerts:

• 1 for Commented-out code
• 1 for For loop variable changed in body
• 1 for Sign check of bitwise operation

[lgtm-com]

This pull request introduces 3 alerts when merging 3e4f166 into 955e971 - view on LGTM.com

new alerts:

• 1 for Commented-out code
• 1 for For loop variable changed in body
• 1 for Sign check of bitwise operation

[lgtm-com]

This pull request introduces 3 alerts when merging 2fb4062 into 955e971 - view on LGTM.com

new alerts:

• 1 for Commented-out code
• 1 for For loop variable changed in body
• 1 for Sign check of bitwise operation

[psolstice]

GUI still have problems, mnemonic confirmation screen feels like it's mandatory although it's not

[lgtm-com]

This pull request introduces 3 alerts when merging 0ec3fe6 into 955e971 - view on LGTM.com

new alerts:

• 1 for Commented-out code
• 1 for For loop variable changed in body
• 1 for Sign check of bitwise operation

[lgtm-com]

This pull request introduces 3 alerts when merging b0f6cb4 into 0398e70 - view on LGTM.com

new alerts:

• 1 for Commented-out code
• 1 for For loop variable changed in body
• 1 for Sign check of bitwise operation

[lgtm-com]

This pull request introduces 3 alerts when merging 7f9f1c4 into 0398e70 - view on LGTM.com

new alerts:

• 1 for Commented-out code
• 1 for For loop variable changed in body
• 1 for Sign check of bitwise operation

[lgtm-com]

This pull request introduces 3 alerts when merging 424dfa4 into 0398e70 - view on LGTM.com

new alerts:

• 1 for Commented-out code
• 1 for For loop variable changed in body
• 1 for Sign check of bitwise operation