…e equal, even when they are

Backport v2/Client.Remove and File.Sync

In `*File.ReadFrom`, ensure that the buffer is filled to capacity (`maxPacket`
length) before performing the write.

Prior to this change, the amount of data read into the buffer was dictated by
the `io.Reader`'s `Read` implementation, and write performance would suffer
when the Read would return less than maxPacket bytes of data. An example
source would be a `net/http` response `Body` from a TLS server, which
seems to cap each read to 16384 bytes -- half the default max packet size of
32768 bytes.

perf: ensure full write buffer for *File.ReadFrom