Do orphan block checking differently... (#655)

... Don't have special knowledge of "blockid" in ProjectSerializer, which should
be very low-level.

Instead, we can deserialize the project first, and use the block ids collected
by the sample block factory since f137a1e.

src/SqliteSampleBlock.cpp

@@ -129,6 +129,8 @@ public:
 
    ~SqliteSampleBlockFactory() override;
 
+   SampleBlockIDs GetActiveBlockIDs() override;
+
    SampleBlockPtr DoGet(SampleBlockID sbid) override;
 
    SampleBlockPtr DoCreate(samplePtr src,
@@ -171,6 +173,21 @@ SampleBlockPtr SqliteSampleBlockFactory::DoCreate(
    return sb;
 }
 
+auto SqliteSampleBlockFactory::GetActiveBlockIDs() -> SampleBlockIDs
+{
+   SampleBlockIDs result;
+   for (auto end = mAllBlocks.end(), it = mAllBlocks.begin(); it != end;) {
+      if (it->second.expired())
+         // Tighten up the map
+         it = mAllBlocks.erase(it);
+      else {
+         result.insert( it->first );
+         ++it;
+      }
+   }
+   return result;
+}
+
 SampleBlockPtr SqliteSampleBlockFactory::DoCreateSilent(
    size_t numsamples, sampleFormat srcformat )
 {