cache: filter out tables that are not requested

Do not fetch table content for list commands that specify a
table name, e.g.

 # nft list table filter

This speeds up listing of a given table by not populating the
cache with tables that are not needed.

 - Full ruleset (huge with ~100k lines).

 # sudo nft list ruleset &> /dev/null
 real    0m3,049s
 user    0m2,080s
 sys     0m0,968s

- Listing per table is now faster:

 # nft list table nat &> /dev/null
 real    0m1,969s
 user    0m1,412s
 sys     0m0,556s

 # nft list table filter &> /dev/null
 real    0m0,697s
 user    0m0,478s
 sys     0m0,220s

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1326
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

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