4 letter word commands leaking memory

[nylar]

Running a 4 letter word command against zetcd causes a spike in memory usage, this applies to all but ruok.

Steps to reproduce:

1. echo cons | nc 127.0.0.1 2181
2. Observe memory usage spike to around 2gb (depending on command, see below).

Looks like the issue is caused by

zetcd/encode.go

Line 244 in 8db3822

blen := int(binary.BigEndian.Uint32(buf[:4]))

where the 4 letter command is incorrectly interpreted as the length of the payload.

So in the case of cons, int(binary.BigEndian.Uint32(buf[:4])) returns 1668247155, this exceeds the capacity of the buffer so a new buffer is allocated of that size causing the large memory usage.