Fix cannot initialize pd if using ipv6 address

[birdstorm]

Previously: if the pd is using an ipv6 address, HostAndPort cannot resolve address correctly.

[birdstorm]

/run-all-tests

[birdstorm]

/run-all-tests

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@c891da3). Click here to learn what that means.
The diff coverage is 81.25%.

@@            Coverage Diff            @@
##             master     #587   +/-   ##
=========================================
  Coverage          ?   52.74%           
  Complexity        ?      956           
=========================================
  Files             ?      135           
  Lines             ?     6404           
  Branches          ?      760           
=========================================
  Hits              ?     3378           
  Misses            ?     2720           
  Partials          ?      306

Impacted Files	Coverage Δ	Complexity Δ
...nt/src/main/java/com/pingcap/tikv/codec/Codec.java	90.36% <ø> (ø)	2 <0> (?)
...rc/main/java/com/pingcap/tikv/TiConfiguration.java	53.75% <100%> (ø)	17 <0> (?)
...main/java/com/pingcap/tikv/util/RangeSplitter.java	65.78% <100%> (ø)	19 <0> (?)
...ient/src/main/java/com/pingcap/tikv/TiSession.java	68.35% <66.66%> (ø)	15 <0> (?)
...lient/src/main/java/com/pingcap/tikv/PDClient.java	84.17% <75%> (ø)	36 <5> (?)

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[zhexuany]

LGTM