Simplify SHAMapNodeID:

The existing SHAMapNodeID object has both a valid and an invalid state
and requirs callers to verify the state of an instance prior to using
it. A simple set of changes removes that restriction and ensures that
all instances are valid, making the code more robust.

This change also:

1. Introduces a new function to construct a SHAMapNodeID from a
   serialized blob; and
2. Reduces the amount of constructors the class exposes.

src/test/shamap/FetchPack_test.cpp

@@ -25,6 +25,7 @@
 #include <ripple/beast/xor_shift_engine.h>
 #include <ripple/protocol/digest.h>
 #include <ripple/shamap/SHAMap.h>
+#include <ripple/shamap/SHAMapSyncFilter.h>
 #include <functional>
 #include <stdexcept>
 #include <test/shamap/common.h>