Make robust limiter utility function

This release contains changes to internal limit/concurrency logic.

Non-breaking changes

• add limiter utility function 1511c24
• make map function internally maintained 0dca9ae
• convert map values to use limiter e24432f
• add code coverage metrics e8e7052
• add callback tests eec9fa8

Breaking changes

• convert each to use map 163da47
  • Each now invokes its task function with (value, key) rather than (value).
  • Each now supports being called with an object rather than just an array
• remove unnecessary p-* deps 3f783c1
  • removes eachSeries should call each with limit 1 for series
  • removes mapSeries should call map with limit 1 for series