MEDIUM: cache: "show cache" on the cli

MEDIUM: cache: "show cache" on the cli

The cli command "show cache" displays the status of the cache, the first
displayed line is the shctx informations with how much blocks available
blocks it contains (blocks are 1k by default).

The next lines are the objects stored in the cache tree, the pointer,
the size of the object and how much blocks it uses, a refcount for the
number of users of the object, and the remaining expiration time (which
can be negative if expired)

Example:

    $ echo "show cache" | socat - /run/haproxy.sock
    0x7fa54e9ab03a: foobar (shctx:0x7fa54e9ab000, available blocks:3921)
    0x7fa54ed65b8c (size: 43190 (43 blocks), refcount:2, expire: 2)
    0x7fa54ecf1b4c (size: 45238 (45 blocks), refcount:0, expire: 2)
    0x7fa54ed70cec (size: 61622 (61 blocks), refcount:0, expire: 2)
    0x7fa54ecdbcac (size: 42166 (42 blocks), refcount:1, expire: 2)
    0x7fa54ec9736c (size: 44214 (44 blocks), refcount:2, expire: 2)
    0x7fa54eca28ec (size: 46262 (46 blocks), refcount:2, expire: -2)