BUG/MINOR: cache: fix "show cache" output

BUG/MINOR: cache: fix "show cache" output

The "show cache" command used to dump the header for each entry into into
the handler loop, making it repeated every ~16kB of output data. Additionally
chunk_appendf() was used instead of chunk_printf(), causing the output to
repeat already emitted lines, and the output size to grow in O(n^2). It used
to take several minutes to report tens of millions of objects from a small
cache containing only a few thousands. There was no more impact though.

This fix must be backported to 1.8.