Simplify and document region life cycle

[aaronbee]

• Removed RegionInfo.SetClient. The client is now set to nil by MarkUnavailable, and then set again with MarkAvailable. This removes bugs and races where a region info may have its Client set when it shouldn't be. A region now has 3 distinct states it may be in:

1. Unavailable: AvailabilityChan() is non-nil and Client() is nil
2. Available: AvailabilityChan() is nil and Client() is non-nil
3. Stale: AvailabilityChan() is nil and Client() is nil

• Documented RegionInfo type with information about those 3 states and the lifecycle.

• Documented establishRegion with what it does and what it's requirements are.

• Removed calls to SetClient(nil)/MarkUnavailable in clientRegionCache methods. That work is the responsibility of the caller.

• getRegionAndClientForRPC no longer marks a region as unavailable if Client is nil after waiting on the AvailabilityChan. If Client is nil, that indicates the region is stale and a lookup has to be performed. Marking it as unavailable and trying to reestablish it is wasted effort.

• The above change broke the use of the meta and admin regions, which are created when creating the gohbase Client/AdminClient, but don't have a Client set. These regions were relying on a missing Client to cause the region to be established. Now the meta and admin regions are established as part of NewClient and NewAdminClient, respectively.

closes: #186

[codecov-commenter]

Codecov Report

Patch coverage: 53.12% and project coverage change: +0.03% :tada:

Comparison is base (4bda353) 70.24% compared to head (288ab9b) 70.27%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #227      +/-   ##
==========================================
+ Coverage   70.24%   70.27%   +0.03%     
==========================================
  Files          27       27              
  Lines        3714     3701      -13     
==========================================
- Hits         2609     2601       -8     
+ Misses        988      986       -2     
+ Partials      117      114       -3     

Files Changed	Coverage Δ
hrpc/call.go	70.00% <ø> (ø)
region/info.go	66.66% <0.00%> (-0.60%)	:arrow_down:
rpc.go	85.54% <36.84%> (+1.02%)	:arrow_up:
admin_client.go	53.57% <100.00%> (+1.21%)	:arrow_up:
caches.go	85.58% <100.00%> (-0.27%)	:arrow_down:
client.go	69.27% <100.00%> (+0.65%)	:arrow_up:

... and 1 file with indirect coverage changes

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[aaronbee]

This change still needs some work.