Increment channel reference count on mmap copy.

Fixes #360.
Increment reference count by channel offset.

o Adds new pfs_channel_addref function. mmap constructors now add a reference.
o Move deleting the channel reference to the mmap destructor.
Increment channel reference count on mmap copy.

Fixes #360.

* ccl/pull/415/head:
  Increment reference count by channel offset.
  Increment channel reference count on mmap copy.