Merge #13243: Make reusable base class for auxiliary indices

ec3073a274 index: Move index DBs into index/ directory. (Jim Posen)
89eddcd365 index: Remove TxIndexDB from public interface of TxIndex. (Jim Posen)
2318affd27 MOVEONLY: Move BaseIndex to its own file. (Jim Posen)
f376a49241 index: Generalize logged statements in BaseIndex. (Jim Posen)
61a1226d87 index: Extract logic from TxIndex into reusable base class. (Jim Posen)
e5af5fc6fb db: Make reusable base class for index databases. (Jim Posen)
9b0ec1a7f9 db: Remove obsolete methods from CBlockTreeDB. (Jim Posen)

Pull request description:

  This refactors most of the logic in TxIndex into a reusable base class for other indices. There are two commits moving code between files, which may be be more easily reviewed using `git diff --color-moved` (https://blog.github.com/2018-04-05-git-217-released/).

  The motivation for this is to support BIP 157 by indexing block filters.

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Tree-SHA512: 0857f04df2aa920178dab2eb8e57984d8eb4d5010deca9971190358479e05b6672ccca2a08af0a7ac9fe02afb947be84cf35a3693204d0667263c6add2959cbf

src/test/txindex_tests.cpp

@@ -15,7 +15,7 @@ BOOST_AUTO_TEST_SUITE(txindex_tests)
 
 BOOST_FIXTURE_TEST_CASE(txindex_initial_sync, TestChain100Setup)
 {
-    TxIndex txindex(MakeUnique<TxIndexDB>(1 << 20, true));
+    TxIndex txindex(1 << 20, true);
 
     CTransactionRef tx_disk;
     uint256 block_hash;