Fix: module cache delta overflow

[dynamite-bud]

This PR makes a fix to the module cache.

In the function below we see that we have three points:

1. We get the current platform time
2. We get the read the cache
3. We read the data getting the when_cached and then subtract it from the current platform time aka now

pub fn get_webc(&self, webc: &str, runtime: &dyn WasiRuntime) -> Option<BinaryPackage> {
        let name = webc.to_string();
        let now = platform_clock_time_get(Snapshot0Clockid::Monotonic, 1_000_000).unwrap() as u128; // -------- 1️⃣ 

        // Fast path
        {
            let cache = self.cache_webc.read().unwrap();  // -------- 2️⃣ 
            if let Some(data) = cache.get(&name) {
                if let Some(when_cached) = data.when_cached.as_ref() {
                    let delta = now - *when_cached;  // -------- 3️⃣ 
                    if delta <= self.cache_time.as_nanos() {
                        return Some(data.clone());
                    }
           ...

Issue:

Assuming two threads reading on same webc

Actions Series
|
| ---> 1️⃣ ----> 2️⃣ ---🔒(on cache_webc) ------------------------ 🔓 (on cache_webc and update when_cached) ------ 3️⃣ --- 🟥
| ---> 1️⃣ ----> 2️⃣ ----^--🕐 (wait for lock release) ---------^ 🔒 (on cache_webc and read updated when_cached) ----- 3️⃣ --- 🟥
|.....................................................................................................................Fails as when_cached is greater than now ^
________________________________________________________________________________________________________________________ Time

So this failure results in a value overflow in 3️⃣ and causes a crash in the tokio_runtime and thus fails the IP tests in wasmer_deploy_testing

[theduke]

LGTM.

Congratulations on your first Wasmer/WASIX bugfix, @dynamite-bud !