core: Miscellaneous transfer timeout improvements

core: Miscellaneous transfer timeout improvements

  * When removing a transfer from the flying_transfers list, only
    rearm the timerfd if the transfer being removed was the first
    transfer *and* had a timeout. This is the only case where the
    timerfd should be altered by a transfer being removed.

  * When searching the flying_transfers list for the next timeout,
    searching can cease when the first transfer with an infinite
    timeout is encountered.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>