feature: add support to config dns client cache size

[chuxi]

for each client key type (tcplocal, tcpremote, udplocal, udpremote), the cache client size is set 5. we can not configure the dns cache size dynamically. If the size is too small, the dns client will create new connections and discard the old ones which are still waiting for dns responses. And it leads to timeout and slow down the dns service.

So in the PR, it add a new configurable parameter for dns local client_cache_size, the default value is 5.

In my own openwrt device, it works much better.