Fix JSON parser handling for certain objects (#7162)

This PR also adds some comments and simplifies
the code. Primary handling is done to ensure
that we make sure to honor cached buffer.

Added unit tests as well

Fixes #7141

pkg/s3select/json/data/7.json

@@ -0,0 +1,3 @@
+{"name":"Michael", "age": 31}
+{"name":"Andy", "age": 30}
+{"name":"Justin", "age": 19}