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

Cherry-picked from: d314e8a

src/txmempool.cpp

@@ -10,6 +10,7 @@
 #include "clientversion.h"
 #include "consensus/consensus.h"
 #include "consensus/validation.h"
+#include "optional.h"
 #include "validation.h"
 #include "policy/policy.h"
 #include "policy/fees.h"
@@ -179,9 +180,9 @@ 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++) {
-            txiter piter = mapTx.find(tx.vin[i].prevout.hash);
+            Optional<txiter> piter = mapTx.find(tx.vin[i].prevout.hash);
             if (piter != mapTx.end()) {
-                parentHashes.insert(piter);
+                parentHashes.insert(*piter);
                 if (parentHashes.size() + 1 > limitAncestorCount) {
                     errString = strprintf("too many unconfirmed parents [limit: %u]", limitAncestorCount);
                     return false;

src/txmempool.h

@@ -18,6 +18,7 @@
 #include "coins.h"
 #include "crypto/siphash.h"
 #include "indirectmap.h"
+#include "optional.h"
 #include "primitives/transaction.h"
 #include "sync.h"
 #include "random.h"

src/util.h

@@ -17,6 +17,7 @@
 
 #include "compat.h"
 #include "fs.h"
+#include "optional.h"
 #include "tinyformat.h"
 #include "utiltime.h"
 
@@ -62,7 +63,7 @@ extern const char * const BITCOIN_PID_FILENAME;
  */
 inline std::string _(const char* psz)
 {
-    boost::optional<std::string> rv = translationInterface.Translate(psz);
+    Optional<std::string> rv = translationInterface.Translate(psz);
     return rv ? (*rv) : psz;
 }
 

src/wallet/coincontrol.h

@@ -6,6 +6,7 @@
 #ifndef BITCOIN_WALLET_COINCONTROL_H
 #define BITCOIN_WALLET_COINCONTROL_H
 
+#include "optional.h"
 #include "primitives/transaction.h"
 #include "dogecoin-fees.h"