Allow to_timestamp(float8) to convert float infinity to timestamp infinity.

Allow to_timestamp(float8) to convert float infinity to timestamp infinity.

With the original SQL-function implementation, such cases failed because
we don't support infinite intervals.  Converting the function to C lets
us bypass the interval representation, which should be a bit faster as
well as more flexible.

Vitaly Burovoy, reviewed by Anastasia Lubennikova