Add 'pread'-backed method for thread-safe hunk reading

[xakep666]

Addressing proposal in #34

This PR adds method chd_read_threadsafe - a thread-safe version of chd_read. This method will be helpful for implementing threaded read-aheads purely by consumers without adding external dependencies (like pthread) .

My main programming language is Go and there's a popular approach "leave concurrency to a caller" used when concurrent code is written. And I think it fits good in this case.

Some implementation details:

• chd_read_threadsafe method has extra argument compressed_buffer which is used to store raw compressed data read from file. This is the one of two things required for thread-safety. chd_read uses same buffer stored in chd_file structure.
• Extra function pointer (interface method) fpread added to chd_core_file_callbacks to allow thread-safe reading from file. Used only in chd_read_threadsafe.
• On *nix libc's pread is being utilized for thread-safe reading from file
• On Windows ReadFile used for same purpose. Reference: https://github.com/aleitner/windows_pread
• On other platforms or in case fpread was not provided chd_read_threadsafe will return error CHDERR_NOT_SUPPORTED.
• hunk_read_into_memory has been tweaked a bit to accept arbitrary compressed data buffer pointer and different reading methods: seek_and_read (old one) or read_at (new one).
• Lazy allocation of internal compressed data buffer chd->compressed as it only used inside chd_read.