Add a configurable randomness source

[rhernandez35]

I had two goals for this change:

1. Expose the configuration option without exposing any new classes or
   interfaces. We should be in strict control of possible randomness
   sources so we can guarantee that they actually do provide random values
   and that they provide randomness efficiently. I didn't want it to be
   possible for users to tank performance by inadvertently using a single
   Random instance.
2. Make the random source configurable and scoped to each
   AWSXRayRecorder instance. I didn't want to introduce any more global
   shared state, so this unfortunately meant I wasn't able to remove the
   global SecureRandom usage in two places: TraceId#fromString and
   DefaultThrowableSerializationStrategy#describeInContext. The latter is
   at least aware of the local segment context and will try to use the
   random generator from its creator when possible, but the former is
   unresolvable without either adding more global shared state or exposing
   the RandomGenerator class.

This commit adds the following public APIs:

• AWSXRayRecorder#useFastRandomGenerator and
  AWSXRayRecorder#useSecureRandomGenerator: for configuring the behavior
  of each AWSXRayRecorder instance.
• AWSXRayRecorderBuilder#useFastRandomGenerator and
  AWSXRayRecorderBuilder#useSecureRandomGenerator: same as above
• ThrowableSerializationStrategy#describeInContext(SegmentContext, Throwable, List<Subsegment>): a new method that allows the current
  segment context to be provided to ThrowableSerializationStrategy. This
  is currently only useful for getting the ID generator from the current
  segment's creator, but I can imagine there being other uses for this
  later.

I unfortunately had to add two internal-only public methods to
AWSXRayRecorder for actually getting random values from its configured
random generator. Adding methods there allowed me to keep the
RandomGenerator class hidden from users.

There are some other minor changes in this PR:

• Removed quadratic String concatenation in Entity#generateId
• SegmentContextResolverChain no longer double-resolves contexts

Issue #, if available:
#216

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[willarmiros]

Thanks again for the changes! Left some thoughts and we should prob wait to hear from @anuraaga before getting this in.

[anuraaga]

Sorry for the late review - left a high level comment about the internal API. Thanks a lot for the help @rhernandez35!

[rhernandez35]

OK, I moved the random generators to internal and added a getter in AWSXRayRecorder. Everything else is mostly the same as in the last revision.

[willarmiros]

Couple more comments

[willarmiros]

LGTM, will wait on @anuraaga to approve and merge. Thanks for your contribution and patience @rhernandez35!

[anuraaga]

Thanks @rhernandez35!