Increase the input block size for bgzip. #1768
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Commit e495718 changed bgzip from unix raw POSIX read() calls to hread(). Unfortunately hread gets its buffer size from stat of the input file descriptor, which can be 4kb for a pipe. We're reading 0xff00 bytes, so this ends up being split over two reads mostly, with one or both involving additional memcpys. This makes the buffered I/O worse performing than non-buffered. In the most extreme cases (cat data | bgzip -l0 > /dev/null) this is a two fold slow down.
The easy solution is just to increase the buffer size to something sensible. It's a little messy as we have to use hfile_internal.h to get hfile_set_blksize, but it works. I'm not sure why we didn't elect to make that API more public. Probably simply out of caution.
Fixes #1767