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.

src/main.cpp

@@ -340,6 +340,10 @@ int main(int argc, char* argv[])
 
 	p2pool::destroy_crypto_cache();
 
+#ifdef WITH_INDEXED_HASHES
+	p2pool::indexed_hash::cleanup_storage();
+#endif
+
 	p2pool::log::stop();
 
 	uv_loop_close(uv_default_loop());