Stop using derived accounts

[moshthepitt]

Using derived accounts for orders and subscriptions make the whole system less scalable.

[moshthepitt]

Leaning towards:

1. merchant accounts should be derived from a simple user-supplied seed
2. order accounts should not be derived but should be regular accounts
3. subscription accounts should be derived using a key that is a hash of merchant pubkey + subscription name
4. the hash function to be used should probably be murmur3
   • we don't need a cryptographic hash function
   • murmur3 does not use rand so can work in bpf unlike other hash functions e.g. xxhash
   • murmur3 has a quick and easy js implementation