Return thread handles from new runThreads

Re-implement the logic of `run` as a function that returns the handles
which can then be joined by the caller.
Most usage sites require a bit of thought with respect to the
order of operations, so `GossipService.run` remains as a wrapper
around the new logic.
This also makes it take a `message_allocator`.

src/cmd/cmd.zig

@@ -376,6 +376,7 @@ fn gossip() !void {
         &.{},
     );
     defer gossip_service.deinit();
+
     try runGossipWithConfigValues(&gossip_service);
 }
 
@@ -571,8 +572,16 @@ fn initRepair(
 }
 
 fn runGossipWithConfigValues(gossip_service: *GossipService) !void {
+    // TODO: use better allocator, unless GPA becomes more performant.
+    var gp_message_allocator: std.heap.GeneralPurposeAllocator(.{}) = .{};
+    defer _ = gp_message_allocator.deinit();
+
     const gossip_config = config.current.gossip;
-    return gossip_service.run(gossip_config.spy_node, gossip_config.dump);
+    return gossip_service.run(.{
+        .message_allocator = gp_message_allocator.allocator(),
+        .spy_node = gossip_config.spy_node,
+        .dump = gossip_config.dump,
+    });
 }
 
 /// determine our shred version and ip. in the solana-labs client, the shred version

src/gossip/fuzz.zig

@@ -308,7 +308,13 @@ pub fn run() !void {
         .noop,
     );
 
-    var fuzz_handle = try std.Thread.spawn(.{}, GossipService.run, .{ &gossip_service_fuzzer, true, false });
+    const fuzz_handle = try std.Thread.spawn(.{}, GossipService.run, .{
+        &gossip_service_fuzzer, .{
+            .message_allocator = allocator,
+            .spy_node = true,
+            .dump = false,
+        },
+    });
 
     const SLEEP_TIME = 0;
     // const SLEEP_TIME = std.time.ns_per_ms * 10;