LOAD DATA transfer plugins

LOAD DATA currently supports reading from files and from the client (LOAD DATA LOCAL).

There's a certain interest in the ability to load data from more sources, in particular from AWS S3, but also from http[s]. This could also enable us to handle compressed files that is using any of the compression format the server supports.

We'll solve it by abstracting file reading code into the plugin. Initially there will be two plugins, file and "local".

The syntax

LOAD { DATA | XML } [ LOCAL ] INFILE ...

will be generalized to

LOAD { DATA | XML } [ plugin ] INFILE ...

We might need some kind of plugin-specific syntax extension for LOAD, so that AWS plugin would be able to specify the credentials. Or may be not, if everything can be part of the "filename", like in http://user:password@host.name/path/to/file

Preferably it should work for SELECT ... INTO OUTFILE too.