refactor: Replace all uses of boost::optional with our own Optional type

After this:

- `boost::optional` is no longer used directly (only through `Optional`
    which is an alias for it)
- `boost/optional.hpp` is only included in one place

src/txmempool.cpp

@@ -8,6 +8,7 @@
 #include <consensus/consensus.h>
 #include <consensus/tx_verify.h>
 #include <consensus/validation.h>
+#include <optional.h>
 #include <validation.h>
 #include <policy/policy.h>
 #include <policy/fees.h>
@@ -155,7 +156,7 @@ bool CTxMemPool::CalculateMemPoolAncestors(const CTxMemPoolEntry &entry, setEntr
         // GetMemPoolParents() is only valid for entries in the mempool, so we
         // iterate mapTx to find parents.
         for (unsigned int i = 0; i < tx.vin.size(); i++) {
-            boost::optional<txiter> piter = GetIter(tx.vin[i].prevout.hash);
+            Optional<txiter> piter = GetIter(tx.vin[i].prevout.hash);
             if (piter) {
                 parentHashes.insert(*piter);
                 if (parentHashes.size() + 1 > limitAncestorCount) {
@@ -860,11 +861,11 @@ const CTransaction* CTxMemPool::GetConflictTx(const COutPoint& prevout) const
     return it == mapNextTx.end() ? nullptr : it->second;
 }
 
-boost::optional<CTxMemPool::txiter> CTxMemPool::GetIter(const uint256& txid) const
+Optional<CTxMemPool::txiter> CTxMemPool::GetIter(const uint256& txid) const
 {
     auto it = mapTx.find(txid);
     if (it != mapTx.end()) return it;
-    return boost::optional<txiter>{};
+    return Optional<txiter>{};
 }
 
 CTxMemPool::setEntries CTxMemPool::GetIterSet(const std::set<uint256>& hashes) const