Wallet Recovery from zcashd HD Seeds

When Sapling was released, zcashd implemented HD derivation of Sapling addresses in a fashion that was inconsistent with HD derivation according to BIP 44. In version 4.7.0 zcashd introduced HD derivation from a mnemonic seed according to BIP 32 and BIP 44, with a nonstandard accommodation in the generation of the mnemonic seed to make it possible to also reproduce previously derived Sapling keys. This accommodation needs to be documented, along with the process for correct discovery of such previously-derived Sapling keys.

In addition, in order to continue allow zcashd's legacy transparent APIs such as getnewaddress and z_getnewaddress to continue to function, zcashd introduced the idea of the ZCASH_LEGACY_ACCOUNT constant for use in address derivation consistent with the previous semantics of those methods. Derivation of keys under ZCASH_LEGACY_ACCOUNT is also nonstandard with respect to BIP 32 and BIP 44, and so needs to be properly documented here in order to make it possible for other wallet implementations to correctly rediscover funds controlled by keys derived using this mechanism.