Reduce memory usage of CBlockIndex

Ported code from https://github.com/zcash/zcash/pull/6192 with various changes needed
for the Hush codebase.

src/chain.cpp

@@ -19,12 +19,57 @@
  ******************************************************************************/
 
 #include "chain.h"
+#include "main.h"
+#include "txdb.h"
 
 using namespace std;
 
 /**
  * CChain implementation
  */
+void CBlockIndex::TrimSolution()
+{
+    AssertLockHeld(cs_main);
+
+    // We can correctly trim a solution as soon as the block index entry has been added
+    // to leveldb. Updates to the block index entry (to update validity status) will be
+    // handled by re-reading the solution from the existing db entry. It does not help to
+    // try to avoid these reads by gating trimming on the validity status: the re-reads are
+    // efficient anyway because of caching in leveldb, and most of them are unavoidable.
+    if (HasSolution()) {
+        std::vector<unsigned char> empty;
+        nSolution.swap(empty);
+    }
+}
+
+CBlockHeader CBlockIndex::GetBlockHeader() const
+{
+    AssertLockHeld(cs_main);
+
+    CBlockHeader header;
+    header.nVersion             = nVersion;
+    if (pprev) {
+        header.hashPrevBlock    = pprev->GetBlockHash();
+    }
+    header.hashMerkleRoot       = hashMerkleRoot;
+    // Hush does not have this, maybe some day
+    // header.hashBlockCommitments = hashBlockCommitments;
+    header.nTime                = nTime;
+    header.nBits                = nBits;
+    header.nNonce               = nNonce;
+    if (HasSolution()) {
+        header.nSolution        = nSolution;
+    } else {
+        CDiskBlockIndex dbindex;
+        if (!pblocktree->ReadDiskBlockIndex(GetBlockHash(), dbindex)) {
+            LogPrintf("%s: Failed to read index entry", __func__);
+            throw std::runtime_error("Failed to read index entry");
+        }
+        header.nSolution        = dbindex.GetSolution();
+    }
+    return header;
+}
+
 void CChain::SetTip(CBlockIndex *pindex) {
     lastTip = pindex;
     if (pindex == NULL) {