Fix "method_complexity" issue in Sources/MistKit/MKDatabase.swift

[leogdion]

Function perform has a Cognitive Complexity of 7 (exceeds 5 allowed). Consider refactoring.

https://codeclimate.com/github/brightdigit/MistKit/Sources/MistKit/MKDatabase.swift#issue_5f581421ee3aa40001000089