extract min & max depth onto coin control (Amiti Uttarwar)
Pull request description:
- Refactor `AvailableCoins` to pull min & max depths from coin control.
- Add `m_max_depth` to coin control to support this.
- Addresses issue https://github.com/bitcoin/bitcoin/issues/15823, see thread for further details.
https://github.com/bitcoin/bitcoin/pull/15906/commits/80ba4241a6773590f6b2c18dae758097b5adc02e
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Backport of Core PR15906