fix(logging): catch and log exceptions in Watchdog runnable to ensure it never stops running

[justinuang]

See https://stackoverflow.com/questions/6894595/scheduledexecutorservice-exception-handling for details

This is the equivalent of googleapis/java-bigtable-hbase#2529

[codecov]

Codecov Report

Merging #1036 into master will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##             master    #1036      +/-   ##
============================================
- Coverage     78.66%   78.65%   -0.02%     
- Complexity     1169     1170       +1     
============================================
  Files           204      204              
  Lines          5189     5195       +6     
  Branches        417      417              
============================================
+ Hits           4082     4086       +4     
- Misses          933      935       +2     
  Partials        174      174              

Impacted Files	Coverage Δ	Complexity Δ
...src/main/java/com/google/api/gax/rpc/Watchdog.java	79.67% <66.66%> (-0.67%)	12.00 <2.00> (+1.00)	⬇️

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[igorbernstein2]

lgtm

[justinuang]

Do you mind merging this? Also, Java 7 seems to be failing for unrelated reasons in the build. The other recent open PR has this problem too: #1032