[app/router] [features/routing] Refine resolution strategy and SkipDNSResolve handling

[Vigilans]

This PR is a continuation of https://github.com/v2fly/v2ray-core/pull/1973. Unlike the base PR it contains changes to existing features API, so created as a separated PR.

This PR removes GetSkipDNSResolve() method from routing.Context interface, and adds a new Unwrap() method to interface for intelligent controlling of routing behavior.

SkipDNSResolve is moved out of routing.Context because compared with other data fields, it works more like a control flag. For separation of control and data channel, it is moved out of routing.Context and specially treated during the creation of ContextWithDNSClient.

ContextWithDNSClient and ContextWithOriginalDestination now returns a second boolean flag, indicating whether the returned context is different from the received context. If:

• For ResolvableContext, the SkipDNSResolve is set in the context to wrap.
• For OriginDestContext, the session.Content does not contain original destination IP.

Then the received original context is directly returned, with the second updated flag set to false. So:

• By returning the original context, no IP resolving capability is added to context.
• By knowing the context didn't get updated, the second round of routing rules traversal (from IPIfNonMatch) could just be skipped.

[codecov-commenter]

Codecov Report

Base: 38.88% // Head: 38.79% // Decreases project coverage by -0.08% :warning:

Coverage data is based on head (82f0059) compared to base (2e0ea88). Patch coverage: 33.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1975      +/-   ##
==========================================
- Coverage   38.88%   38.79%   -0.09%     
==========================================
  Files         616      617       +1     
  Lines       36336    36386      +50     
==========================================
- Hits        14128    14117      -11     
- Misses      20630    20684      +54     
- Partials     1578     1585       +7     

Impacted Files	Coverage Δ
app/router/command/config.go	96.07% <0.00%> (-3.93%)	:arrow_down:
common/session/session.go	70.58% <ø> (ø)
features/routing/session/context_origindest.go	0.00% <0.00%> (ø)
app/router/config.pb.go	12.86% <5.26%> (-0.44%)	:arrow_down:
infra/conf/synthetic/router/router.go	64.64% <37.50%> (-5.24%)	:arrow_down:
app/dispatcher/default.go	29.71% <39.13%> (+0.30%)	:arrow_up:
features/routing/dns/context.go	38.46% <42.85%> (-2.72%)	:arrow_down:
app/router/router.go	28.93% <56.52%> (+1.34%)	:arrow_up:
features/routing/session/context.go	67.27% <100.00%> (+0.60%)	:arrow_up:
app/router/command/errors.generated.go	0.00% <0.00%> (-100.00%)	:arrow_down:
... and 7 more

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