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/wallet/coinselection.cpp

@@ -4,11 +4,10 @@
 
 #include <wallet/coinselection.h>
 
+#include <optional.h>
 #include <util/system.h>
 #include <util/moneystr.h>
 
-#include <boost/optional.hpp>
-
 // Descending order comparator
 struct {
     bool operator()(const OutputGroup& a, const OutputGroup& b) const
@@ -219,7 +218,7 @@ bool KnapsackSolver(const CAmount& nTargetValue, std::vector<OutputGroup>& group
     nValueRet = 0;
 
     // List of values less than target
-    boost::optional<OutputGroup> lowest_larger;
+    Optional<OutputGroup> lowest_larger;
     std::vector<OutputGroup> applicable_groups;
     CAmount nTotalLower = 0;