[feature] add support for multiple accounts

[mirceanis]

The SDK should support more than the Uport.defaultAccount

There should exist a method of listing all the accounts, for ex:
fun Uport.allAccounts(): List<Account>

Distinction has to be made between root addresses and addresses derived from the same seed.
It's probably necessary to store the derivation path(s) as fields of Account objects.

• disable single account limitation
• add isDefault field to Account class
• isolate account storage behind an interface with upsert(acc), get(handle), delete(handle)
• save new accounts in account storage
• load accounts from acc storage
• expose allAccounts()

[mirceanis]

This issue is obsolete