Use a TTL cache for tx validation in DefaultTransactionValidator

Fixes #997

Author: zixuanzh

setup.py

@@ -42,6 +42,7 @@
         "web3==4.3.0",
         # required for rlp>=1.0.0
         "eth-account>=0.2.1,<1",
+        "cachetools==2.1.*",
     ],
     'test': [
         "hypothesis==3.44.26",

trinity/tx_pool/validators.py

@@ -1,3 +1,5 @@
+import cachetools.func
+
 from typing import (
     Type
 )
@@ -45,6 +47,7 @@ def __call__(self, transaction: BaseTransactionFields) -> bool:
         else:
             return True
 
+    @cachetools.func.ttl_cache(maxsize=1024, ttl=300)
     def get_appropriate_tx_class(self) -> Type[BaseTransaction]:
         current_tx_class = self.chain.get_vm_class().get_transaction_class()
         # If the current head of the chain is still on a fork that is before the currently