[Feature] Conan 2 cache concurrency

[memsharded]

This ticket is to gather and centralize all the related tickets.
There are 3 different aspects to make the cache concurrent:

• conan config install and other commands that can change the Conan home configuration concurrently
• Package installation and building concurrently by different processes
• Better management of "dirty" status to recover gently from crashes and interrupted operations (already a feature, but requires refactoring and alignment with the concurrency)

Out of the scope:

• Concurrency at multi-machine or multi-OS level. The planned supported concurrency is at the machine level, from the same OS. OS sync mechanism file-locks will be used, and that is only guaranteed to work at the OS level, no shared or mounted drives
• Sharing the same cache among different Conan versions. The same Conan version must be used for the same cache.

Conan config install concurrent:

• [bug] Concurrent conan config install with Git fails #6233
• Conan config install failing randomly for parallel builds.  #15018

Package concurrency

• [feature] Add an ability to build dependencies in parallel #14570
• [bug] kill/crash conan process leaves a lock file containing count -1 #11033
• [question] Random install failures? #8241
• [bug] Concurrency - installing same package from multiple workers seems to generate races #6753
• export-pkg lacks synchronization #4648
• Race condition while multiple conan processes are installing the same package #5505
• [bug] Interrupted install leaves locks in bad state #7181