parse_timestamp: accept RFC3339-style timezone and %FT%R[:%S[.%N]]

parse_timestamp: accept RFC3339-style timezone and %FT%R[:%S[.%N]]

We basically parsed the RFC3339 format already, except with a space:
      NOTE: ISO 8601 defines date and time separated by "T".
      Applications using this syntax may choose, for the sake of
      readability, to specify a full-date and full-time separated by
      (say) a space character.
so now we handle both
  2012-11-23 11:12:13.456
  2012-11-23T11:12:13.456
as equivalent.

Parse directly-suffixed Z and +05:30 timezones as well:
  2012-11-23T11:12:13.456Z
  2012-11-23T11:12:13.456+02:00
as they're both defined by RFC3339.

We do /not/ allow z or t; the RFC says
      NOTE: Per [ABNF] and ISO8601, the "T" and "Z" characters in this
      syntax may alternatively be lower case "t" or "z" respectively.

      This date/time format may be used in some environments or contexts
      that distinguish between the upper- and lower-case letters 'A'-'Z'
      and 'a'-'z' (e.g. XML).  Specifications that use this format in
      such environments MAY further limit the date/time syntax so that
      the letters 'T' and 'Z' used in the date/time syntax must always
      be upper case.  Applications that generate this format SHOULD use
      upper case letters.
We /are/ in a case-sensitive environment, neither are in wide-spread
use, and "z" poses an issue of whether "todayz" should be the same
as "todayZ" ("today UTC") or an error (it should be an error).

Fractional seconds are limited to six digits (they're nominally
   time-secfrac    = "." 1*DIGIT
), since we only support 1µs-resolution timestamps, and limit to six
digits in our other sub-second formats.

Parsing
  2012-11-23T11:12
is an extension two ways (no seconds, no timezone),
mirroring our "canonical" format.

Fixes #5194