Reduce the memory overhead of Bugsnag when session tracking isn't enabled

[imjoehaines]

Goal

This PR requires the 'concurrent' gem only when it's needed, to help reduce the memory overhead of Bugsnag when session tracking isn't enabled (by ~4 MiB)

The shared SessionTracker instance is created on demand, so if an application doesn't use session tracking the 'concurrent' gem isn't required at all

This did have one drawback though — moving the require from the top level to the initialize method meant that the overhead from the concurrent gem was shifted into the first request, as that's where the first session will start. This had quite a big impact on response time (from ~10ms to ~70ms locally) so we now create the session tracker upfront. We do this after configuration has run so that we can avoid creating it if session tracking is disabled

Tests

The existing test suite should confirm this works

I've also manually tested that the SessionTracker isn't initialized when sessions are disabled (config.auto_capture_sessions = false)

Linked issues

Fixes #536

Review

For the submitter, initial self-review:

• Commented on code changes inline explain the reasoning behind the approach
• Reviewed the test cases added for completeness and possible points for discussion
• A changelog entry was added for the goal of this pull request
• Check the scope of the changeset - is everything in the diff required for the pull request?
• This pull request is ready for:
  • Initial review of the intended approach, not yet feature complete
  • Structural review of the classes, functions, and properties modified
  • Final review

For the pull request reviewer(s), this changeset has been reviewed for:

• Consistency across platforms for structures or concepts added or modified
• Consistency between the changeset and the goal stated above
• Internal consistency with the rest of the library - is there any overlap between existing interfaces and any which have been added?
• Usage friction - is the proposed change in usage cumbersome or complicated?
• Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
• Concurrency concerns - if components are accessed asynchronously, what issues will arise
• Thoroughness of added tests and any missing edge cases
• Idiomatic use of the language