De-duplicate tx hashes and pub keys to save memory (off by default) (#382)

P2Pool-main: 8.2 MB saved
P2Pool-mini: 66 MB saved
P2Pool-nano: 25.2 MB saved

The feature is available only when building from source and is intended for use on low-memory systems (for example, a VPS server with < 1 GB RAM).

It only makes sense to use with `--no-cache --no-randomx` in the command line because cache and RandomX hasher take much more memory.

CMakeLists.txt

@@ -18,6 +18,7 @@ option(WITH_LTO "Use link-time compiler optimization (if linking fails for you,
 option(WITH_UPNP "Include UPnP support. If this is turned off, p2pool will not be able to configure port forwarding on UPnP-enabled routers." ON)
 option(WITH_GRPC "Include gRPC support. If this is turned off, p2pool will not be able to merge mine with Tari." ON)
 option(WITH_TLS "Include TLS support. If this is turned off, p2pool will not support Stratum TLS connections, and lack many other security features. It's recommended to keep it ON!" ON)
+option(WITH_INDEXED_HASHES "Save memory used for storing transaction hashes and public keys (a bit slower block verification - use it only if you compile for a very low memory system)" OFF)
 
 option(WITH_MERGE_MINING_DONATION "Merge mine donations to the author. This doesn't affect your hashrate or payouts in any way - only unused merge mining capacity will be utilised. If you merge mine yourself, your settings will take priority." ON)
 
@@ -190,6 +191,11 @@ if (WITH_TLS)
 	set(SOURCES ${SOURCES} src/tls.cpp)
 endif()
 
+if (WITH_INDEXED_HASHES)
+	add_compile_definitions(WITH_INDEXED_HASHES)
+	set(SOURCES ${SOURCES} src/indexed_hash.cpp)
+endif()
+
 source_group(TREE "${CMAKE_CURRENT_SOURCE_DIR}" PREFIX "Header Files" FILES ${HEADERS})
 source_group(TREE "${CMAKE_CURRENT_SOURCE_DIR}" PREFIX "Source Files" FILES ${SOURCES})