Scripting: simpler reply buffer creation in luaRedisGenericCommand().

It if faster to just create the string with a single sdsnewlen() call.
If c->bufpos is zero, the call will simply be like sdsemtpy().
Define HAVE_ATOMIC for clang.
Scripting: luaRedisGenericCommand() fast path for buffer-only replies.

When the reply is only contained in the client static output buffer, use
a fast path avoiding the dynamic allocation of an SDS string to
concatenate the client reply objects.
Scripting: replace tolower() with faster code in evalGenericCommand().

The function showed up consuming a non trivial amount of time in the
profiler output. After this change benchmarking gives a 6% speed
improvement that can be consistently measured.
Fixed missing c->bufpos reset in luaRedisGenericCommand().

Bug introduced when adding a fast path to avoid copying the reply buffer
for small replies that fit into the client static buffer.
Scripting: cache argv in luaRedisGenericCommand().

~ 4% consistently measured speed improvement.
Scripting: don't call lua_gc() after Lua script run.

Calling lua_gc() after every script execution is too expensive, and
apparently does not make the execution smoother: the same peak latency
was measured before and after the commit.

This change accounts for scripts execution speedup in the order of 10%.
Scripting: Use faster API for Lua client c->argv creation.

Replace the three calls to Lua API lua_tostring, lua_lua_strlen,
and lua_isstring, with a single call to lua_tolstring.

~ 5% consistent speed gain measured.
Scripting: objects caching for Lua c->argv creation.

Reusing small objects when possible is a major speedup under certain
conditions, since it is able to avoid the malloc/free pattern that
otherwise is performed for every argument in the client command vector.
Test: handle new osx 'leaks' error.

Sometimes the process is still there but no longer in a state that can
be checked (after being killed). This used to happen after a call to
SHUTDOWN NOSAVE in the scripting unit, causing a false positive.
Scripting test: check that Lua can call commands rewirting argv.

SPOP, tested in the new test, is among the commands rewritng the
client->argv argument vector (it gets rewritten as SREM) for command
replication purposes.

Because of recent optimizations to client->argv caching in the context
of the Lua internal Redis client, it is important to test for SPOP to be
callable from Lua without bad effects to the other commands.
REDIS_ENCODING_EMBSTR_SIZE_LIMIT set to 39.

The new value is the limit for the robj + SDS header + string +
null-term to stay inside the 64 bytes Jemalloc arena in 64 bits
systems.