wp_get_http should stream directly to the file

[rmccue]

Right now, wp_get_http does a regular HTTP request via WP_Http, then saves out to a file. This necessitates loading the entire file into memory, then saving it out, which can massively increase the memory usage. This is important in imports (the only place I can see where the function is used).

Instead, we should use WP_Http's native file streaming functionality, which pushes the bytes into the file as we go, rather than loading into memory.

In addition, I'd propose we deprecate wp_get_http, and anything using it (notably WordPress Importer) should use the normal HTTP functions.

[rmccue]

Also:

• The function calls set_time_limit (#21521)
• The function manually handles redirection for HEAD requests, noting "WP_HTTP no longer follows redirects for HEAD requests", but that's not actually true. WP_Http simply changes the *default* to 0 for HEAD requests, but the redirection setting still applies. (I suspect this was changed, but wp_get_http wasn't kept up-to-date)

[rmccue]

Use direct file streaming, and deprecate the function

[rmccue]

Added patch, including a deprecated message.

[SergeyBiryukov]

Duplicate of #33709.