common/buffer: add function bufferlist::claim_append_piecewise(list& bl).

common/buffer: add function  bufferlist::claim_append_piecewise(list&  bl).

This only useful for bl is bufferlist::page_aligned_appender. Using
this function can remove memcopy for continue ptrs.
Because page_aligned_appender::flush will split a ptr into two or
more ptrs. For this case, rebuild_aligned_size_and_memory can't handle,
it  will rebuild.

For example
a=bl.get_page_aligned_appender(1);
a.append(3K)
a.flush();
t.claim_append(bl);
a.append(1K);
a.flush();
t.claim_append(bl);

dst.claim_append(t);
//3K and 1K ptr are continue in memory. But they are two ptrs..
dst.is_aligned_size_and_memory(4096,4096) is false.

We add new function claim_append_piecewise() to specially
handle this case.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>