isisd: fix multiple values display in show isis database detail json

isisd: fix multiple values display in show isis database detail json

Multiple occurrences of the same (sub-)TLVs are mixed into a single JSON
object. Last values replaces the previous.

Display all the occurrences in an array for the following (sub-)TLVs:
- is-reach
- ip-reach
- ext-reach
- ext-ip-reach
- ipv6-reach
- MT
- prefix-SID

Fixes: a2cac12a63 ("isisd: Add json to show isis database command.")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>