large integers in JSON should be strings

[allenan]

JS doesn't have a native way to handle large integers.

Numeric literals with absolute values equal to 2^53 or greater are too large to be represented accurately as integers

for timestamps with ms precision such as in the witness_info object, the convention is to return them as strings and let the client decide how to turn those into big integers using a library such as biginteger.js for example.

[madninja]

Note that in the fix above this will only happen for newly inserted witnesses @allenan