Add a "tx output spender" index

Adds an outpoint -> txid index, which can be used to find which transactions spent a given output.
We use a composite key with 2 parts (suggested by @romanz): hash(spent outpoint) and tx position, with an empty value.
To find the spending tx for a given outpoint, we do a prefix search (prefix being the hash of the provided outpoint), and for all keys that match this prefix
we load the tx at the position specified in the key and return it, along with the block hash, if does spend the provided outpoint.
To handle reorgs we just erase the keys computed from the removed block.

This index is extremely useful for Lightning and more generally for layer-2 protocols that rely on chains of unpublished transactions.
If enabled, this index will be used by `gettxspendingprevout` when it does not find a spending transaction in the mempool.

src/node/caches.cpp

@@ -7,6 +7,7 @@
 #include <common/args.h>
 #include <common/system.h>
 #include <index/txindex.h>
+#include <index/txospenderindex.h>
 #include <kernel/caches.h>
 #include <logging.h>
 #include <node/interface_ui.h>
@@ -22,6 +23,8 @@
 static constexpr size_t MAX_TX_INDEX_CACHE{1024_MiB};
 //! Max memory allocated to all block filter index caches combined in bytes.
 static constexpr size_t MAX_FILTER_INDEX_CACHE{1024_MiB};
+//! Max memory allocated to tx spenderindex DB specific cache in bytes.
+static constexpr size_t MAX_TXOSPENDER_INDEX_CACHE{1024_MiB};
 //! Maximum dbcache size on 32-bit systems.
 static constexpr size_t MAX_32BIT_DBCACHE{1024_MiB};
 
@@ -44,6 +47,8 @@ CacheSizes CalculateCacheSizes(const ArgsManager& args, size_t n_indexes)
     IndexCacheSizes index_sizes;
     index_sizes.tx_index = std::min(total_cache / 8, args.GetBoolArg("-txindex", DEFAULT_TXINDEX) ? MAX_TX_INDEX_CACHE : 0);
     total_cache -= index_sizes.tx_index;
+    index_sizes.txospender_index = std::min(total_cache / 8, args.GetBoolArg("-txospenderindex", DEFAULT_TXOSPENDERINDEX) ? MAX_TXOSPENDER_INDEX_CACHE : 0);
+    total_cache -= index_sizes.txospender_index;
     if (n_indexes > 0) {
         size_t max_cache = std::min(total_cache / 8, MAX_FILTER_INDEX_CACHE);
         index_sizes.filter_index = max_cache / n_indexes;

src/node/caches.h

@@ -21,6 +21,7 @@ namespace node {
 struct IndexCacheSizes {
     size_t tx_index{0};
     size_t filter_index{0};
+    size_t txospender_index{0};
 };
 struct CacheSizes {
     IndexCacheSizes index;