TODO: 1.8 Modified buffer size approach

 1.5 get rid of PATH_MAX

 1.6 progress callback without doubles

 1.7 Happy Eyeball dual stack connect

 1,8 Modified buffer size approach

 2. libcurl - multi interface

 2.1 More non-blocking

    http://tools.ietf.org/html/rfc6555

1.8 Modified buffer size approach

 Current libcurl allocates a fixed 16K size buffer for download and an

 additional 16K for upload. They are always unconditionally part of the easy

 handle. If CRLF translations are requested, an additional 32K "scratch

 buffer" is allocated. A total of 64K transfer buffers in the worst case.

 First, while the handles are not actually in use these buffers could be freed

 so that lingering handles just kept in queues or whatever waste less memory.

 Secondly, SFTP is a protocol that needs to handle many ~30K blocks at once

 since each need to be individually acked and therefore libssh2 must be

 allowed to send (or receive) many separate ones in parallel to achieve high

 transfer speeds. A current libcurl build with a 16K buffer makes that

 impossible, but one with a 512K buffer will reach MUCH faster transfers. But

 allocating 512K unconditionally for all buffers just in case they would like

 to do fast SFTP transfers at some point is not a good solution either.

 Dynamically allocate buffer size depending on protocol in use in combination

 with freeing it after each individual transfer? Other suggestions?

2. libcurl - multi interface

2.1 More non-blocking