need to add a `RwLock` variant to `stratum_apps::custom_mutex::Mutex`

[plebhash]

on #83 I proposed to completely remove stratum_apps::custom_mutex::Mutex, but got some pushback together with the acknowledgement that a RwLock variant to Mutex could be a good improvement

so this issue aims to replace #83

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it's important to note that using RwLock everywhere is not necessarily the most performant strategy

whenever the R/W ratio is "balanced", the performance overhead of RwLock is not justified and a Mutex will be more performant (todo @Shourya742 share ref)

but if there's way more Reads than Writes, then RwLock is justified