[Refactor] Remove GetInputAge and GetMasternodeInputAge

[random-zebra]

Introduce a GetCoinDepth() utility function in CCoinsViewCache and use that instead (with pcoinsTip... no need to lock/load the mempool too in the backend).
This should fix the invalid lock order:

2020-09-02 10:23:17 POTENTIAL DEADLOCK DETECTED
2020-09-02 10:23:17 Previous lock order was:
2020-09-02 10:23:17  cs_process_message /src/masternodeman.cpp:690 (in thread pivx-msghand)
2020-09-02 10:23:17  (1) mempool.cs /src/main.cpp:777 (in thread pivx-msghand)
2020-09-02 10:23:17  (2) cs_main /src/main.cpp:785 (in thread pivx-msghand)
2020-09-02 10:23:17 Current lock order is:
2020-09-02 10:23:17  (2) cs_main /src/main.cpp:4571 (in thread )
2020-09-02 10:23:17  (1) cs /src/txmempool.cpp:590 (in thread )

To be able to use the new per-txout interface, this is based on top of:

• [Core] Per-txout model for chainstate database #1801

The present PR starts with [Refactoring] Add GetCoinDepth utility function to CCoinsViewCache (294e177)

BONUS: remove the cached "ages" saved/serialized in CMasternode class... no comment.

[furszy]

Strong ACK 2be200d ☕ with a refactor comment.

[random-zebra]

Rebased on master and squashed the name change.

[furszy]

utACK cf7003b

[furszy]

extra: tested with 1829 and all good. Merging..