Fixing CPU Profiling and Tracing

[nisdas]

Resolves #549

• This was done by adding a context, to debug.Exit() , so that profiling and tracing would only be stopped if the flags had been enabled previously.
• For resolving the second part of the issue, a check for a canceled context was added so that the beacon service routines in the beacon package would not loop endlessly even though the rpc service had been shut down.
• Modify tests to maintain 100% test coverage in the beacon package.

[codecov]

Codecov Report

Merging #550 into master will increase coverage by 0.41%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master     #550      +/-   ##
==========================================
+ Coverage   75.93%   76.35%   +0.41%     
==========================================
  Files           9       42      +33     
  Lines         798     2906    +2108     
==========================================
+ Hits          606     2219    +1613     
- Misses        118      482     +364     
- Partials       74      205     +131

Impacted Files	Coverage Δ
beacon-chain/node/node.go	59.14% <0%> (ø)	⬆️
validator/beacon/service.go	93.27% <100%> (ø)
validator/node/node.go	45.87% <50%> (ø)
shared/p2p/service.go	67.01% <0%> (ø)
beacon-chain/utils/clock.go	100% <0%> (ø)
beacon-chain/types/active_state.go	75.47% <0%> (ø)
beacon-chain/rpc/service.go	92.42% <0%> (ø)
validator/attester/service.go	100% <0%> (ø)
validator/params/config.go	100% <0%> (ø)
... and 26 more

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[terencechain]

Nice! Straightforward fix. Thanks for doing this!