BUG: masterUncollated readStream causes checkOut/checkIn (#2952)

- updating the header information (by copying) was closing the stream,
  removing all watches and doing a checkOut/checkIn, which could lead to
  dangling references.
  Now just close the stream and simply copy the IOobject header
  information directly.

STYLE: mark regIOobject assignment operator as possibly deprecated

- will revisit to revise or remove in the future