Backports the remaining six commits from PR13033, thus activating the
new transaction index database:
9b2704777 [doc] Include txindex changes in the release notes.
ed77dd6b3 [test] Simple unit test for TxIndex.
6d772a3d4 [rpc] Public interfaces to GetTransaction block until synced.
a03f804f2 [index] Move disk IO logic from GetTransaction to TxIndex::FindTx.
e0a3b8003 [validation] Replace tx index code in validation code with TxIndex.
8181db88f [init] Initialize and start TxIndex in init code.
Also incorporates fixes from PR14071 and PR15410:
faf4a9b67 qa: Stop txindex thread before calling destructor
fab6b07c1 test: txindex: interrupt threadGroup before calling destructor
Depends on D2904