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.

.github/workflows/c-cpp.yml

@@ -429,8 +429,8 @@ jobs:
     strategy:
       matrix:
         config:
-          - {c: "gcc", cxx: "g++", flags: "-ffunction-sections -Wno-error=maybe-uninitialized -Wno-error=attributes -Wno-unknown-attributes"}
-          - {c: "clang", cxx: "clang++", flags: "-fuse-ld=lld -Wno-unused-command-line-argument -Wno-nan-infinity-disabled -Wno-unknown-attributes"}
+          - {c: "gcc", cxx: "g++", flags: "-ffunction-sections -Wno-error=maybe-uninitialized -Wno-error=attributes -Wno-attributes"}
+          - {c: "clang", cxx: "clang++", flags: "-fuse-ld=lld -Wno-unused-command-line-argument -Wno-nan-infinity-disabled -Wno-attributes"}
 
     defaults:
       run: