[build] Move policy settings to new src/policy/settings unit

This moves the following policy settings functions and globals to a new
src/policy/settings unit in lib_server:

- `incrementalRelayFee`
- `dustRelayFee`
- `nBytesPerSigOp`
- `fIsBareMultisigStd`

These settings are only required by the node and should not be accessed
by other libraries.

src/interfaces/chain.cpp

@@ -13,6 +13,7 @@
 #include <policy/fees.h>
 #include <policy/policy.h>
 #include <policy/rbf.h>
+#include <policy/settings.h>
 #include <primitives/block.h>
 #include <primitives/transaction.h>
 #include <protocol.h>

src/interfaces/node.cpp

@@ -20,6 +20,7 @@
 #include <policy/feerate.h>
 #include <policy/fees.h>
 #include <policy/policy.h>
+#include <policy/settings.h>
 #include <primitives/block.h>
 #include <rpc/server.h>
 #include <scheduler.h>