Finish implementation of copy method.

All types (besides EntropyVal) now support a native copy operation,
which uses primitives of the underlying datatypes to perform a quick
copy, without serialization.

EntropyVal is the one exception - since that type is rather complex
(many members) and will probably not be copied a lot, if at all, it
makes sense to just use the serialization function.

This will have to be slightly re-written in the near-term-future to use
the new serialization function for that opaque type.

This change also introduces a new x509_from_der bif, which allows to
parse a der into an opaque of x509.

This change removes the d2i_X509_ wrapper function; this was a remnant
when d2i_X509 took non-const arguments. We directly use d2i_X509 at
several places assuming const-ness, so there does not seem to ba a
reason to keep the wrapper.

This change also exposed a problem in the File cache - cases in which an
object was brought back into the cache, and writing occurred in the
file_open event were never correctly handeled as far as I can tell.

src/probabilistic/Topk.cc

@@ -183,6 +183,13 @@ void TopkVal::Merge(const TopkVal* value, bool doPrune)
 		}
 	}
 
+Val* TopkVal::DoClone(CloneState* state)
+	{
+	auto clone = new TopkVal(size);
+	clone->Merge(this);
+	return clone;
+	}
+
 bool TopkVal::DoSerialize(SerialInfo* info) const
 	{
 	DO_SERIALIZE(SER_TOPK_VAL, OpaqueVal);