UploadFileVec uses FileVec instead of ByteVec; most code gets removed.

FileVec can have it's _len set during close.
Remove getChunkSz back to CHUNK_SZ so the compiler gets a constant.
Remove espc() back to _espc so no v-call in inner loops.
Use Arrays.copyOf instead byte-by-byte copy.
Use FileVec.CHUNK_SZ instead of Vec.CHUNK_SZ.
Shrink CHUNK_SZ to an int.
Restore close() behavior.