Add SPI precedence support and apply for transport SPI

[sczyh30]

Describe what this PR does / why we need it

Add @SpiOrder support and apply precedence support for transport SPI (CommandCenter, HeartbeatSender).

Does this pull request fix one issue?

NONE

Describe how you did it

• Add a @SpiOrder annotation to indicate precedence. Also update SpiLoader to support get the SPI with the highest precedence.
• Add precedence support for transport SPI (CommandCenter, HeartbeatSender). Two providers now will choose the instance with the highest precedence.

Describe how to verify it

Run a demo, put with two different transport modules, then see whether the SPI of higher order is activated.

Special notes for reviews

NONE

[codecov-io]

Codecov Report

Merging #675 into master will decrease coverage by 0.2%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master     #675      +/-   ##
============================================
- Coverage     41.04%   40.84%   -0.21%     
- Complexity     1212     1216       +4     
============================================
  Files           267      267              
  Lines          7854     7911      +57     
  Branches       1062     1072      +10     
============================================
+ Hits           3224     3231       +7     
- Misses         4234     4288      +54     
+ Partials        396      392       -4

Impacted Files	Coverage Δ	Complexity Δ
.../java/com/alibaba/csp/sentinel/util/SpiLoader.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...l/cluster/server/connection/ConnectionManager.java	76% <0%> (+2%)	11% <0%> (+1%)	⬆️
...ava/com/alibaba/csp/sentinel/node/ClusterNode.java	100% <0%> (+4.76%)	8% <0%> (+1%)	⬆️
...a/csp/sentinel/slots/statistic/base/LeapArray.java	72.54% <0%> (+4.9%)	35% <0%> (+2%)	⬆️

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[CarpenterLee]

LGTM