Fixed critical memory leak from EVAL.

Multiple missing calls to lua_pop prevented the error handler function
pushed on the stack for lua_pcall() to be popped before returning,
causing a memory leak in almost all the code paths of EVAL (both
successful calls and calls returning errors).

This caused two issues: Lua leaking memory (and this was very visible
from INFO memory output, as the 'used_memory_lua' field reported an
always increasing amount of memory used), and as a result slower and
slower GC cycles resulting in all the CPU being used.

Thanks to Tanguy Le Barzic for noticing something was wrong with his 2.8
slave, and for creating a testing EC2 environment where I was able to
investigate the issue.
Test: added a memory efficiency test.
Test: Lua stack leak regression test added.
Cluster: fixed a bug in clusterSendPublish() due to inverted statements.

The code used to copy the header *after* the 'hdr' pointer was already
switched to the new buffer. Of course we need to do the reverse.
Cluster: use non-blocking I/O for the cluster bus.
Cluster: clusterReadHandler() reworked to be more correct and simpler to follow.
A mistype fixed
mistype fixed
Cluster: always use safe iteartors to iterate server.cluster->nodes.
Cluster: don't save HANDSHAKE nodes in nodes.conf.
Cluster: CLUSTER SAVECONFIG command added.
redis-cli: fix big keys search when the key no longer exist.

The code freed a reply object that was never created, resulting in a
segfault every time randomkey returned a key that was deleted before we
queried it for size.
Cluster: free HANDSHAKE nodes after node_timeout.

Handshake nodes should turn into normal nodes or be freed in a
reasonable amount of time, otherwise they'll keep accumulating if the
address they are associated with is not reachable for some reason.
Cluster: don't add an handshake node for the same ip:port pair multiple times.
Allow AUTH / PING when disconnected from slave and serve-stale-data is no.
Cluster: added time field in cluster bus messages.

The time is sent in requests, and copied back in reply packets.
This way the receiver can compare the time field in a reply with its
local clock and check the age of the request associated with this reply.

This is an easy way to discard delayed replies. Note that only a clock
is used here, that is the one of the node sending the packet. The
receiver only copies the field back into the reply, so no
synchronization is needed between clocks of different hosts.
Cluster: PFAIL -> FAIL transition allowed for slaves.

First change: now there is no need to be a master in order to detect a
failure, however the majority of masters signaling PFAIL or FAIL is needed.

This change is important because it allows slaves rejoining the cluster
after a partition to sense the FAIL condition so that eventually all the
nodes agree on failures.
htonu64() and ntohu64 added to endianconv.h.
Cluster: configEpoch added in cluster nodes description.
Cluster: broadcast currentEpoch and configEpoch in packets header.