Pass pointers rather than interfaces to {get,put}FreeConn
Pass pointers rather than interfaces to {get,put}FreeConn

This should be a bit faster, since the compiler can remove
a few instructions.

Before:

mov    0x7360e0,%rax
xor    %rbp,%rbp
cmp    %rbp,%rax
je     4397f6 <gomemcache/memcache.(*Client).getConn+0x116>
mov    0x50(%rsp),%rbx
mov    %rbx,(%rsp)
mov    0x58(%rsp),%rcx
mov    %rax,0x8(%rsp)
mov    %rcx,0x10(%rsp)
callq  4392c0 <gomemcache/memcache.(*Client).getFreeCon

After:

mov    0x50(%rsp),%rbx
mov    %rbx,(%rsp)
mov    0x58(%rsp),%rbx
mov    %rbx,0x8(%rsp)
callq  439270 <gomemcache/memcache.(*Client).getFreeConn>

rainycape/memcache,brunoqc/memcache,axiomzen/memcache