wallet: cache descriptor ID to avoid repeated descriptor string creation

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.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/wallet/scriptpubkeyman.cpp b/src/wallet/scriptpubkeyman.cpp
index bc3327cdb2..d2b2801aa8 100644
--- a/src/wallet/scriptpubkeyman.cpp
+++ b/src/wallet/scriptpubkeyman.cpp
@@ -2601,7 +2601,7 @@ std::unique_ptr<CKeyMetadata> DescriptorScriptPubKeyMan::GetMetadata(const CTxDe
 uint256 DescriptorScriptPubKeyMan::GetID() const
 {
     LOCK(cs_desc_man);
-    return DescriptorID(*m_wallet_descriptor.descriptor);
+    return m_wallet_descriptor.id;
 }
 
 void DescriptorScriptPubKeyMan::SetCache(const DescriptorCache& cache)
@@ -2655,7 +2655,7 @@ bool DescriptorScriptPubKeyMan::AddCryptedKey(const CKeyID& key_id, const CPubKe
 bool DescriptorScriptPubKeyMan::HasWalletDescriptor(const WalletDescriptor& desc) const
 {
     LOCK(cs_desc_man);
-    return m_wallet_descriptor.descriptor != nullptr && desc.descriptor != nullptr && m_wallet_descriptor.descriptor->ToString() == desc.descriptor->ToString();
+    return !m_wallet_descriptor.id.IsNull() && !desc.id.IsNull() && m_wallet_descriptor.id == desc.id;
 }
 
 void DescriptorScriptPubKeyMan::WriteDescriptor()