channelstorage: Rename callbacks that conflict with DEBUG_FD_LEAKS.

channelstorage: Rename callbacks that conflict with DEBUG_FD_LEAKS.

DEBUG_FD_LEAKS replaces calls to "open" and "close" with functions that keep
track of file descriptors, even when those calls are actually callbacks
defined in structures like ast_channelstorage_instance->open and don't touch
file descriptors.  This causes compilation failures.  Those callbacks
have been renamed to "open_instance" and "close_instance" respectively.

Resolves: #1287