Upgrade dashmap to 4.0.0-rc4 to fix deadlock

The map would deadlock if we initialized it with more than a certain
number of items on startup (on my machine, more than 15 or 16 items).
The latest RC fixes this issue:
xacrimon/dashmap#81 (comment)

Cargo.toml

@@ -12,7 +12,7 @@ async-graphql-warp = "1.4.10"
 async-trait = "0.1.31"
 chrono = { version = "0.4.11", features = ["serde"] }
 circular-queue = "0.2.4"
-dashmap = "4.0.0-rc3"
+dashmap = "4.0.0-rc4"
 escaper = "0.1.0"
 futures = "0.3.5"
 http = "0.2.1"

src/main.rs

@@ -54,6 +54,7 @@ async fn main() -> anyhow::Result<()> {
     bosses_to_request_hashes_for
         .iter()
         .for_each(|boss| hash_inbox.request_hash_for_boss(boss));
+    tokio::spawn(hash_worker);
 
     // Cleanup task that periodically:
     // * removes bosses that haven't been seen in a while
@@ -132,10 +133,6 @@ async fn main() -> anyhow::Result<()> {
     let server = tokio::spawn(warp::serve(routes).try_bind(bind_addr));
 
     tokio::select! {
-        // For some reason the `hash_worker` doesn't work consistently if started with `tokio::spawn`
-        _ = hash_worker => {
-            slog::error!(log, "Hash updater ended unexpectedly");
-        }
         _ = twitter_worker => {
             slog::error!(log, "Disconnected from Twitter stream");
         }