getncchanges.c: Refactor to track more state using repl_chunk

getncchanges.c: Refactor to track more state using repl_chunk

To prepare GET_TGT to deal with a large number of links better, there
is now a 'repl_chunk' struct to help keep track of all the factors
relating to the current chunk of replication data (i.e. how many
objects/links we can send and how many we've already processed). This
means we can have a consistent way of working out whether the current
chunk is full (whether that be due to objects, links, or just too much
time taken).

This patch should not alter functionality. This is just a refactor to
add the basic framework, which will be used in the next patch.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>