perf: add column name index mapping in PostgresqlRow class

[cty123]

[resolves #636]

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Issue description

Previously getting the fields of the row results requires looping the fields array list to find the field with the specified name, this can be very slow when there's a large amount of columns retrieved. Here I am trying to add a cache to map the column name to index such that we only need to loop the fields list once.

New Public APIs

Additional context