wallet: skip block scan if block was created before wallet birthday

To avoid wasting processing power, we can skip blocks that occurred
before the wallet's creation time,  since these blocks are guaranteed
not to contain any relevant wallet data.

This has direct implications (an speed improvement) on the underlying
blockchain synchronization process as well.

The reason is that the validation interface queue is limited to
10 tasks per time. This means that no more than 10 blocks can be
waiting for the wallet(s) to be processed while we are synchronizing
the chain (activating the best chain to be more precise).
Which can be a bottleneck if blocks arrive and are processed faster
from the network than what they are  processed by the wallet(s).

src/wallet/scriptpubkeyman.cpp

@@ -710,6 +710,8 @@ void LegacyScriptPubKeyMan::UpdateTimeFirstKey(int64_t nCreateTime)
     } else if (!nTimeFirstKey || nCreateTime < nTimeFirstKey) {
         nTimeFirstKey = nCreateTime;
     }
+
+    NotifyFirstKeyTimeChanged(this, nTimeFirstKey);
 }
 
 bool LegacyScriptPubKeyMan::LoadKey(const CKey& key, const CPubKey &pubkey)
@@ -2736,6 +2738,8 @@ void DescriptorScriptPubKeyMan::UpdateWalletDescriptor(WalletDescriptor& descrip
     m_map_script_pub_keys.clear();
     m_max_cached_index = -1;
     m_wallet_descriptor = descriptor;
+
+    NotifyFirstKeyTimeChanged(this, m_wallet_descriptor.creation_time);
 }
 
 bool DescriptorScriptPubKeyMan::CanUpdateToWalletDescriptor(const WalletDescriptor& descriptor, std::string& error)