refactor: pcoinsTip -> CChainState::CoinsTip()

This aliasing makes subsequent commits easier to review; eventually CoinsTip()
will return the CCoinsViewCache managed by CChainState.

test/functional/feature_dbcrash.py

@@ -58,7 +58,7 @@ class ChainstateWriteCrashTest(BitcoinTestFramework):
         self.base_args = ["-limitdescendantsize=0", "-maxmempool=0", "-rpcservertimeout=900", "-dbbatchsize=200000"]
 
         # Set different crash ratios and cache sizes.  Note that not all of
-        # -dbcache goes to pcoinsTip.
+        # -dbcache goes to the in-memory coins cache.
         self.node0_args = ["-dbcrashratio=8", "-dbcache=4"] + self.base_args
         self.node1_args = ["-dbcrashratio=16", "-dbcache=8"] + self.base_args
         self.node2_args = ["-dbcrashratio=24", "-dbcache=16"] + self.base_args