fleet-server performance improvements

[michel-laterman]

In order to better assist scaling efforts we should audit the fleet-server codebase for any performance improvements we can make.

The performance improvements we should seek are:

• Reduce calls to elasticsearch where possible (merging two calls with a painless script is preferred over two separate ES updates)
• Reduce allocations in order to reduce heap size and reduce the work that the garbage collector has to do.
• Ensure struct alignment (easy to add automation to ensure this works)

Acceptance criteria:

• All changes must have benchmarks associated with them.
• Benchmarks should show a reduction in one of the tracked catagories
• Benchmark comparisons between the baseline and improvements - with -memprofile must be available for analysis

Focus will be directed towards high-use packages first.