use weak reference to hold bean objects so that they could be garbage collected

[nobodyiam]

Fixes #1670.

[codecov-io]

Codecov Report

Merging #1676 into master will increase coverage by 0.02%.
The diff coverage is 64.7%.

@@             Coverage Diff             @@
##             master   #1676      +/-   ##
===========================================
+ Coverage     48.77%   48.8%   +0.02%     
- Complexity     1906    1911       +5     
===========================================
  Files           392     392              
  Lines         11492   11524      +32     
  Branches       1196    1204       +8     
===========================================
+ Hits           5605    5624      +19     
- Misses         5458    5464       +6     
- Partials        429     436       +7

Impacted Files	Coverage Δ	Complexity Δ
.../framework/apollo/spring/property/SpringValue.java	87.71% <41.66%> (-12.29%)	20 <5> (+1)
...rk/apollo/spring/property/SpringValueRegistry.java	83.33% <77.27%> (-9.53%)	10 <4> (+5)
.../apollo/internals/RemoteConfigLongPollService.java	77.3% <0%> (-0.62%)	27% <0%> (-1%)

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[coveralls]

Coverage increased (+0.08%) to 52.586% when pulling 790ee89 on nobodyiam:fix-1670 into d93a6ba on ctripcorp:master.

[Reason0701]

http://192.168.56.5:8080/notifications/v2?cluster=default&appId=SampleApp&ip=192.168.1.35&notifications=%5B%7B%22namespaceName%22%3A%22application%22%2C%22notificationId%22%3A-1%7D%5D, reason: Could not complete get operation [Cause: connect timed out]