MINOR: chunk: add chunk_istcat() to concatenate an ist after a chunk

MINOR: chunk: add chunk_istcat() to concatenate an ist after a chunk

We previously relied on chunk_cat(dst, b_fromist(src)) for this but it
is not reliable as the allocated buffer is inside the expression and
may be on a temporary stack. While it's possible to allocate stack space
for a struct and return a pointer to it, it's not possible to initialize
it form a temporary variable to prevent arguments from being evaluated
multiple times. Since this is only used to append an ist after a chunk,
let's instead have a chunk_istcat() function to perform exactly this
from a native ist.

The only call place (URI computation in the cache) was updated.