Add thread manager crate to agave

[alexpyattaev]

Problem

There exist a variety of perf issues related to unorganized thread pools that spawn far more threads than are useful on a given machine, this was identified by firedancer and is relatively easy to fix in agave.

Idea is to eventually address the needs of

• https://github.com/orgs/anza-xyz/projects/3
• Allow configuration of all validator thread-pool sizes via CLI #105

Summary of Changes

Added new agave-thread-manager crate, which is to be gradually hooked into the agave itself. It will be used to centralize thread pool creation such that their core allocations can be controlled, and total thread count can gradually be reduced to match core count as closely as possible. Crate comes with its own benchmarks and tests to establish config policies.