Implement several simple functions in DescriptorScriptPubKeyMan

Summary:
Implements a bunch of one liners: UpgradeKeyMetadata, IsFirstRun, HavePrivateKeys,
KeypoolCountExternalKeys, GetKeypoolSize, GetTimeFirstKey, CanGetAddresses,
RewriteDB

Backport of Core PR16528 [18/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/4cb9b69be031e1dc65d8964794781b347fd948f5

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8416