DateFormatter only provide second level precision for absl format

[wbpcode]

This is because two problems:

1. time_t is used when calling the absl::FormatTime. The time_t only provide second level precision.
2. The time cache. The cache is updated in second level. And the code will get the cached string and replace sub-second part to sub-second of current timepoint. But the code only handle the specifier %f and don't handle absl format (actually it's not easy to handle it, this means we maybe shouldn't use time cache for dynamic format string).

[wbpcode]

cc @dio @mattklein123

[doujiang24]

START_TIME can be customized using a format string.

IIUC, the doc does not mention support the fractional second format in absl::FormatTime, instead it only mention the %f format. So, it's safe to cache by seconds, thoughts?

[wbpcode]

The default format that used by Envoy is %Y-%m-%dT%H:%M:%E3SZ but current DateFormtter even cannot support it correctly.

And the DateFormatter has tell it could support absl::FormatTime in its comments.

https://github.com/envoyproxy/envoy/blob/9537fce85d4f919d4b16de280f0ddf5b0130b8b5/source/common/common/utility.h#L43

So, I am inclined this is a code bug rather than a users-aware bug? If someone want to use this utility class, he may get unexpected result.

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