Add alloc_close_notify config param.

[clia]

When using the turn service, we want to count the number of traffic bytes forwarded by each allocation in the application. I think a good time is when the allocation is closed. So I designed a parameter to pass an mpsc sender in, and send the AllocationInfo out to the application when the allocation is closed.

[codecov]

Codecov Report

Patch coverage: 62.26% and project coverage change: -0.05 ⚠️

Comparison is base (94a779d) 61.50% compared to head (af18f54) 61.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #421      +/-   ##
==========================================
- Coverage   61.50%   61.45%   -0.05%     
==========================================
  Files         537      537              
  Lines       49432    49481      +49     
  Branches    12308    12321      +13     
==========================================
+ Hits        30404    30411       +7     
  Misses       9873     9873              
- Partials     9155     9197      +42     

Impacted Files	Coverage Δ
turn/examples/turn_server_udp.rs	0.00% <0.00%> (ø)
turn/src/server/config.rs	72.72% <0.00%> (ø)
...tion/allocation_manager/allocation_manager_test.rs	60.41% <45.45%> (-1.01%)	⬇️
ice/src/agent/agent_vnet_test.rs	69.69% <100.00%> (-0.12%)	⬇️
ice/src/candidate/candidate_relay_test.rs	76.78% <100.00%> (+0.42%)	⬆️
...e/src/candidate/candidate_server_reflexive_test.rs	73.46% <100.00%> (+0.55%)	⬆️
turn/src/allocation/allocation_test.rs	52.79% <100.00%> (+2.25%)	⬆️
...n/src/allocation/channel_bind/channel_bind_test.rs	46.42% <100.00%> (+1.98%)	⬆️
turn/src/auth/auth_test.rs	67.92% <100.00%> (+0.61%)	⬆️
turn/src/client/client_test.rs	58.62% <100.00%> (+0.35%)	⬆️
... and 2 more

... and 18 files with indirect coverage changes

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