plugin for fetching cache dump from any node after joining legion

This plugin allows to fetch copy of cache content from any node joined to legion.
It allows for example to hot-add new SSL frontend server with already populated session cache (you just need to set lower frequency for legion so it joins faster).

Usage:
```
[uwsgi]
[...]
# define cache
cache2 = name=cache1,items=1000,store=/tmp/lc1.ucache,udp=225.1.1.1:1500,node=225.1.1.1:1500

# define legion as usual
legion = legion1 225.1.1.1:19678 101 bf-cbc:abc
legion-node = legion1 225.1.1.1:19678
legion-join = legion1 cmd:echo legion_joined_hook

# connect legion to cache
legion-cache = legion1 cache1
# set socket address advertised to other nodes using legion scroll
legion-cache-dump-socket = 127.0.0.1:3001
```

Please review.