Don't update node pong time via gossip.

This feature was implemented in the initial days of the Redis Cluster
implementaiton but is not a good idea at all.

1) It depends on clocks to be synchronized, that is already very bad.
2) Moreover it adds a bug where the pong time is updated via gossip so
no new PING is ever sent by the current node, with the effect of no PONG
received, no update of tables, no clearing of PFAIL flag.

In general to trust other nodes about the reachability of other nodes is
a broken distributed programming model.
Only run the fast active expire cycle if master & enabled.
Update server.lastbgsave_status when fork() fails.
DEBUG SDSLEN added.

This command is only useful for low-level debugging of memory issues due
to sds wasting memory as empty buffer at the end of the string.
Don't over-allocate the sds string for large bulk requests.

The call to sdsMakeRoomFor() did not accounted for the amount of data
already present in the query buffer, resulting into over-allocation.
tryObjectEncoding(): don't call stringl2() for too big strings.

We are sure that a string that is longer than 21 chars cannot be
represented by a 64 bit signed integer, as -(2^64) is 21 chars:

strlen(-18446744073709551616) => 21
tryObjectEncoding(): optimize sds strings if possible.

When no encoding is possible, at least try to reallocate the sds string
with one that does not waste memory (with free space at the end of the
buffer) when the string is large enough.