Add a quota handler callback

[rg0now]

Note: the PR is on top of #419, see the changes on top of that PR here.

RFC 8656 includes the following text:

To mitigate either intentional or unintentional denial-of-service attacks against the server by clients with valid usernames and passwords, it is RECOMMENDED that the server impose limits on both the number of allocations active at one time for a given username and on the amount of bandwidth those allocations can use. The server should reject new allocations that would exceed the limit on the allowed number of allocations active at one time with a 486 (Allocation Quota Exceeded [sic!]) [error].

(Note the mistake in the text: the name of the error is in fact "Allocation Quota Reached", not "Allocation Quota Exceeded".)

This PR adds a quota handler callback function which, if specified, is called by the server just before making an allocation for a user. The handler should return a single bool: if true then the allocation request can proceed, otherwise the request is rejected with the 486 (Allocation Quota Reached) error. Then, the lifecycle API can be used to track the number of active allocations per user and this callback can be leveraged to reject allocation requests that would exceed the user's quota.

Note that the other DoS mitigation recommendation given in the RFC (limiting the amount of bandwidth a single user can use) is not targeted by this PR.

[codecov]

Codecov Report

Attention: Patch coverage is 91.13300% with 18 lines in your changes missing coverage. Please review.

Project coverage is 68.74%. Comparing base (cc03474) to head (72d3ed7).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/server/util.go	73.07%	5 Missing and 2 partials ⚠️
internal/allocation/five_tuple.go	60.00%	4 Missing ⚠️
server.go	50.00%	3 Missing and 1 partial ⚠️
internal/server/nonce.go	50.00%	0 Missing and 1 partial ⚠️
relay_address_generator_range.go	0.00%	1 Missing ⚠️
server_config.go	97.43%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #423      +/-   ##
==========================================
+ Coverage   68.15%   68.74%   +0.59%     
==========================================
  Files          43       43              
  Lines        2352     3107     +755     
==========================================
+ Hits         1603     2136     +533     
- Misses        582      804     +222     
  Partials      167      167              

Flag	Coverage Δ
go	68.74% <91.13%> (+0.59%)	⬆️
wasm	25.52% <2.95%> (-2.37%)	⬇️

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