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.

tests/src/crypto_tests.cpp

@@ -26,7 +26,7 @@ namespace p2pool {
 TEST(crypto, derivation)
 {
 	init_crypto_cache();
-
+	{
 	hash pub, sec;
 	generate_keys(pub, sec);
 	ASSERT_TRUE(check_keys(pub, sec));
@@ -105,9 +105,13 @@ TEST(crypto, derivation)
 			}
 		} while (!f.eof());
 	}
-
+	}
 	clear_crypto_cache(0);
 	destroy_crypto_cache();
+
+#ifdef WITH_INDEXED_HASHES
+	indexed_hash::cleanup_storage();
+#endif
 }
 
 }