perf: make m_peer_mutex a SharedMutex

[PastaPastaPasta]

Issue being fixed or feature implemented

m_peer_mutex is a great use case for a shared mutex; it's private, well contained scope, and 99% of usages are for the read case. The only time we hold an exclusive lock is when adding or removing a peer, which happens rarely compared to all other usages.

On latest nightly, this mutex only represents about 2% of contention (all in PeerRef PeerManagerImpl::GetPeerRef), however, I intend to move over mnauth / verified protx related stuff over into PeerManager, and I expect that will result in significantly more contention over m_peer_mutex, making this change more valuable.

I expect this will reduce contention over this mutex to ~0

What was done?

How Has This Been Tested?

Hasn't yet, I will want to deploy a dev version of this and view what happens to contention over this mutex.

Breaking Changes

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation
• I have assigned this pull request to a milestone (for repository code-owners and collaborators only)

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