pool: Decompose the pool and balancer crates

[olix0r]

• Pool::update_pool is replaced with a narrower API:
  • Avoids need for unneeded allocations;
  • Slightly modifies the p2c endpoint update metrics, removing the DNE label.
• Split up crates:
  • Move the pool trait to a minimal crate
  • Move the balancer's P2cPool types into a library crate
• Rename NewBalancePeakEwma to NewBalance

[codecov]

Codecov Report

Merging #2597 (4c15a89) into main (f191eac) will decrease coverage by 0.10%.
Report is 2 commits behind head on main.
The diff coverage is 88.78%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2597      +/-   ##
==========================================
- Coverage   68.06%   67.97%   -0.10%     
==========================================
  Files         330      330              
  Lines       14996    14986      -10     
==========================================
- Hits        10207    10186      -21     
- Misses       4789     4800      +11     

Files	Coverage Δ
linkerd/app/core/src/control.rs	91.39% <100.00%> (ø)
linkerd/app/outbound/src/http/concrete/balance.rs	89.70% <100.00%> (ø)
linkerd/app/outbound/src/opaq/concrete.rs	58.59% <100.00%> (ø)
linkerd/proxy/balance/queue/src/error.rs	66.66% <ø> (ø)
linkerd/proxy/balance/queue/src/failfast.rs	96.92% <ø> (ø)
linkerd/proxy/balance/queue/src/future.rs	71.42% <ø> (ø)
linkerd/proxy/balance/queue/src/lib.rs	100.00% <ø> (ø)
linkerd/proxy/balance/queue/src/message.rs	90.00% <ø> (ø)
linkerd/proxy/balance/queue/src/service.rs	80.64% <ø> (ø)
linkerd/proxy/http/src/lib.rs	100.00% <ø> (ø)
... and 3 more

... and 2 files with indirect coverage changes

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