v2/wallets.ts: combine consolidate/fanout
v2/wallets.ts: combine consolidate/fanout

* Extract common ports of both funcs into `manageUnspents`
* Fix documentation
* Use `this` instead of `self` (with `.bind(this)`)

Issue: BG-14108

BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS