Auto merge of #12406 - izgzhen:refactor-file, r=Manishearth

Refactor FileAPI implementation

Most are simple refactoring, cleanups and improvements, but still involving two slightly notable changes:

+ In `filemanager`, now we read the file content based on requested `RelativePos` by `seek` and `read_exact` (rather than `read_to_end` then do slicing). This strategy might be again adjusted in future performance tuning but certainly better than nothing.
+ Also, I cached more file meta-info in both sides and left a block of comment on `filemanager`'s file reading mentioning the snapshot-state problem (not solved now though).

r? @Manishearth

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