fees: move fee_estimates.dat into fees directory

Move block policy fee estimates from fee_estimates.dat to
fees/block_policy_estimates.dat.

On startup, migrate the legacy file to the new path when only the legacy
file exists. If both files exist, keep the new file and remove the
legacy file.

Rename the block policy estimator args source files to the generic
estimator_args.{cpp,h} names and rename FeeestPath to
BlockPolicyFeeEstPath while the path helper is moved into the shared fee
estimator argument code.

test/functional/test_framework/test_framework.py

@@ -965,6 +965,7 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
                 return os.path.join(cache_node_dir, self.chain, *paths)
 
             os.rmdir(cache_path('wallets'))  # Remove empty wallets dir
+            shutil.rmtree(cache_path('fees'), ignore_errors=True)
             for entry in os.listdir(cache_path()):
                 if entry not in ['chainstate', 'blocks', 'indexes']:  # Only indexes, chainstate and blocks folders
                     os.remove(cache_path(entry))