When memory backend selected, no snap sync (#1738)

nimbus/sync/handlers.nim

@@ -9,12 +9,19 @@
 # except according to those terms.
 
 import
+  ../db/select_backend,
   ./handlers/eth as handlers_eth,
-  ./handlers/setup as handlers_setup,
-  ./handlers/snap as handlers_snap
+  ./handlers/setup as handlers_setup
 
 export
-  handlers_eth, handlers_setup, handlers_snap
+  handlers_eth, handlers_setup
+
+when dbBackend != select_backend.none:
+  import
+    ./handlers/snap as handlers_snap
+
+  export
+    handlers_snap
 
 static:
   type

nimbus/sync/handlers/setup.nim

@@ -12,10 +12,10 @@
 
 import
   eth/p2p,
+  ../../db/select_backend,
   ../../core/[chain, tx_pool],
   ../protocol,
-  ./eth as handlers_eth,
-  ./snap as handlers_snap
+  ./eth as handlers_eth
 
 # ------------------------------------------------------------------------------
 # Public functions: convenience mappings for `eth`
@@ -47,7 +47,11 @@ proc addEthHandlerCapability*(
 # Public functions: convenience mappings for `snap`
 # ------------------------------------------------------------------------------
 
-proc addSnapHandlerCapability*(
+when dbBackend != select_backend.none:
+  import 
+    ./snap as handlers_snap
+    
+  proc addSnapHandlerCapability*(
       node: var EthereumNode;
       peerPool: PeerPool;
       chain = ChainRef(nil);