Move recovery_cache to thread_local

libraries/chain/transaction.cpp

@@ -87,7 +87,7 @@ flat_set<public_key_type> transaction::get_signature_keys( const vector<signatur
    using boost::adaptors::transformed;
 
    constexpr size_t recovery_cache_size = 1000;
-   static recovery_cache_type recovery_cache;
+   static thread_local recovery_cache_type recovery_cache;
    const digest_type digest = sig_digest(chain_id, cfd);
 
    flat_set<public_key_type> recovered_pub_keys;