Move Hashers to util/hasher.{cpp/h}

Move the hashers that we use for hash tables to a common place.

Moved hashers:
- SaltedTxidHasher
- SaltedOutpointHasher
- FilterHeaderHasher
- SignatureCacheHasher
- BlockHasher

src/txmempool.cpp

@@ -1134,5 +1134,3 @@ CTxMemPool::EpochGuard::~EpochGuard()
     ++pool.m_epoch;
     pool.m_has_epoch_guard = false;
 }
-
-SaltedTxidHasher::SaltedTxidHasher() : k0(GetRand(std::numeric_limits<uint64_t>::max())), k1(GetRand(std::numeric_limits<uint64_t>::max())) {}