Right now a wallet descriptor is converted to it's string representation
(via `Descriptor::ToString`) repeatedly at different instances:
- on finding a `DescriptorScriptPubKeyMan` for a given descriptor
(`CWallet::GetDescriptorScriptPubKeyMan`, e.g. used by the
`importdescriptors` RPC); the string representation is created once
for each spkm in the wallet and at each iteration again for
the searched descriptor (`DescriptorScriptPubKeyMan::HasWalletDescriptor`)
- whenever `DescriptorScriptPubKeyMan::GetID()` is called, e.g. in
`TopUp` or any instances where a descriptor is written to the DB
to determine the database key etc.
As there is no good reason to calculate a fixed descriptor's string/ID
more than once, add the ID as a field to `WalletDescriptor` and
calculate it immediately at initialization (or deserialization).
`HasWalletDescriptor` is changed to compare the spkm's and searched
descriptor's ID instead of the string to take use of that.
This speeds up the functional test `wallet_miniscript.py` by a factor of
5-6x on my machine (3m30.95s on master vs. 0m38.02s on PR). The recently
introduced "max-size TapMiniscript" test-case introduced a descriptor
that takes 2-3 seconds to create a string representation, so the
repeated calls to that were significantly hurting the performance.
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