rpc: Use mempool from node context instead of global

Currently they are identical, but in the future we might want to turn
the mempool into a unique_ptr. Replacing the global with the mempool
pointer from the node context simplifies this step.

src/rpc/mining.cpp

@@ -244,6 +244,7 @@ static UniValue getmininginfo(const JSONRPCRequest& request)
             }.Check(request);
 
     LOCK(cs_main);
+    const CTxMemPool& mempool = EnsureMemPool();
 
     UniValue obj(UniValue::VOBJ);
     obj.pushKV("blocks",           (int)::ChainActive().Height());
@@ -290,7 +291,7 @@ static UniValue prioritisetransaction(const JSONRPCRequest& request)
         throw JSONRPCError(RPC_INVALID_PARAMETER, "Priority is no longer supported, dummy argument to prioritisetransaction must be 0.");
     }
 
-    mempool.PrioritiseTransaction(hash, nAmount);
+    EnsureMemPool().PrioritiseTransaction(hash, nAmount);
     return true;
 }
 
@@ -476,6 +477,7 @@ static UniValue getblocktemplate(const JSONRPCRequest& request)
         throw JSONRPCError(RPC_CLIENT_IN_INITIAL_DOWNLOAD, PACKAGE_NAME " is in initial sync and waiting for blocks...");
 
     static unsigned int nTransactionsUpdatedLast;
+    const CTxMemPool& mempool = EnsureMemPool();
 
     if (!lpval.isNull())
     {
@@ -510,7 +512,7 @@ static UniValue getblocktemplate(const JSONRPCRequest& request)
                 if (g_best_block_cv.wait_until(lock, checktxtime) == std::cv_status::timeout)
                 {
                     // Timeout: Check transactions for update
-                    // without holding ::mempool.cs to avoid deadlocks
+                    // without holding the mempool lock to avoid deadlocks
                     if (mempool.GetTransactionsUpdated() != nTransactionsUpdatedLastLP)
                         break;
                     checktxtime += std::chrono::seconds(10);