Support extensible slot chain builder using SPI mechanism

[sczyh30]

Describe what this PR does / why we need it

Support extensible slot chain builder.

Due to the fact that the order between slots in slot chain is essential, we make the slot chain builder extensible so that users can implement their own builder, where they can arrange the order between slots.

Does this pull request fix one issue?

Resolves #55

Describe how you did it

• The SlotChainBuilder is extensible now using Java SPI mechanism. sentinel-core provides the basic implementation DefaultSlotChainBuilder.
• Add a SlotChainProvider to load slot chain builder and create new slot chain. The rule:
  • If there is no user-defined SlotChainBuilder, then use the default builder
  • If user-defined SlotChainBuilder exists, the provider will pick the first loaded slot chain builder, then cache the builder.

Describe how to verify it

Integration test between modules to verify the load order of multi slot chain builders.

[codecov-io]

Codecov Report

Merging #145 into master will increase coverage by 0.16%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##             master     #145      +/-   ##
============================================
+ Coverage     45.72%   45.88%   +0.16%     
- Complexity      556      562       +6     
============================================
  Files           114      115       +1     
  Lines          3823     3844      +21     
  Branches        532      536       +4     
============================================
+ Hits           1748     1764      +16     
- Misses         1860     1863       +3     
- Partials        215      217       +2

Impacted Files	Coverage Δ	Complexity Δ
...re/src/main/java/com/alibaba/csp/sentinel/Env.java	80% <ø> (-3.34%)	1 <0> (ø)
...ba/csp/sentinel/slots/DefaultSlotChainBuilder.java	100% <100%> (ø)	2 <1> (?)
.../src/main/java/com/alibaba/csp/sentinel/CtSph.java	65.9% <100%> (ø)	8 <0> (ø)	⬇️
...baba/csp/sentinel/slotchain/SlotChainProvider.java	63.63% <63.63%> (ø)	5 <5> (?)
...ntinel/slots/statistic/metric/WindowLeapArray.java	80% <0%> (+12%)	6% <0%> (+1%)	⬆️

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[CarpenterLee]

LGTM