This patch adds support to pass continuous aggregate names to the
`set_chunk_time_interval` function to align it with functions, such as
`show_chunks`, `drop_chunks`, and others.
It reuses the previously existing function to find a hypertable or
resolve a continuous aggregate to its underlying hypertable found in
chunk.c. It, however, moves the function to hypertable.c and exports it
from here. There is some discussion if this functionality should stay in
chunk.c, though, it feels wrong in that file now that it is exported.