Add support for a shared-mutex type

We are having new use-cases that could benefit from a shared-mutex type. Examples include but are not limited to:

• SERVER-57811: uses a custom SpinLock implementation to swap the storage engine.
• SERVER-51055: introduced SyncUnique to avoid data-races while setting/getting ServiceContext fields (e.g., fastClockSource).

The absence of a shared-mutex type leaves developers with no option other than introducing new, case-specific shared-mutex-like types.

The desired interface is very similar to std::shared_mutex, but the implementation should address the following:

• The shared path must be very fast, and almost impose no cost.
• Both exclusive and shared accesses must be interruptible (e.g., by accepting a object that implements the Interruptible interface).
• Often, it is okay to delay the exclusive path in favor of the shared path. This should be considered in implementing the shared-mutex type.

The aim of this ticket is to:

• Identify the shared-mutex implementation that fits the above considerations/requirements.
• Wrap the implementation inside a Mongo library under stdx namespace.
• File tickets to replace existing code that is no longer relevant with the introduction of the new shared-mutex type.