[boost] Date_time performance patch

Jeff Garland wrote:

there's a couple small changes for date-time.

I have some performance tweaks for date-time. Basically, tokenizer is replaced with faster token_iterator, convert_to_lower() converts inplace instead of copying. I wonder if boost::algorithm::tolower() should be used instead of convert_to_lower(). Unfortunately I don't have time to prepare the diff yet, because I need to separate the right changes from dirty hacks I used to squeeze some more speed or to support parsing broken date formats. Andrey