NiFi EL: migration from SimpleDateFormat to DateTimeFormatter

In NiFi Expression Language we are commonly using SimpleDateFormat. It is old, inefficient approach: due to mutability of SimpleDateFormat it need to be recreated for each operation. Also this format is lax in some places where user would like to be more strict. In my opinion better approach would be usage of java8 DateTimeFormatter instead.

I've done some benchmarks that you can check on your own in FormatEvaluatorBenchmark. Results on my 8-core, i7-1065G7:

• before change (SimpleDateFormat): 11.230 ± 5.407 us/op
• after switching to DateTimeFormatter API: 4.747 ± 0.426 us/op
• after introduction of preparation of formatter for literal formats: 2.025 ± 0.055 us/op

This change is not 100% transparent so some changes might be necessary in users code. Most of differences are visible in modifications that I've made in TestQuery tests:

• back tick (`) for escaping of extra characters is not supported anymore - only single quote is supported
• "repeated" syntax like "dd" for days strictly check if two digits were provided - if someone need to use more lax syntax, need to use single "d" syntax

[update]
After switching to lenient mode, parsing is compatible with SimpleDateFormat in second point ("repeated" syntax)