The trigger is invoked on IPROTO CALL/EVAL. The trigger callback is
passed a context with function name or eval expression and arguments
(MsgPack data). It will be used for auditing CALL/EVAL events in the EE
version.

NO_TEST=ee
NO_DOC=internal
NO_CHANGELOG=internal

Re-factor box_lua_call and box_lua_eval so that they don't take
call_request. This approach is more scalable: in case of a
functional index, the user expects to see a tuple with field
names so we should be able to pass not only raw msgpack, but
also a tuple to a Lua call so we need an universal way to pass
arguments to _call methods.

To pass a tuple msgpack introduced a new port_msgpack: the port
class with dump_lua method.
A new method get_msgpack returns a content of a port as a
msgpack data. The lifecycle of the returned value is
implementation-specific: it may either be returned directly from
the port, in which case the data will stay alive as long as the
port is alive, or it may be allocated on the fiber()->gc, in
which case the caller is responsible for cleaning up.

Needed for #4182, #1260

Closes #2946.

@TarantoolBot document
Title: fixed module reload
There was a bug in tarantool documentation:
https://tarantool.io/en/doc/1.7/book/box/
box_schema/#lua-function.box.schema.func.reload
Now it is allowed to reload all functions in loadable
module via one method. Legacy method including finction
name is forbidden.
box.schema.func.reload("utils")       -- ok since now
box.schema.func.reload("utils.func1") -- forbidden since now

Global reload is still unsupported because it seems
to be useless.
box.schema.func.reload()              -- invalid!

The iproto subsystem switches between two output buffers once in a while
in order to reclaim memory so passing a pointer to the output buffer
directly to box_process_call() or box_process_eval() is incorrect in
case the called function yields. To fix that, let's make these functions
return the CALL/EVAL result in a port object, which then can then be
encoded in msgpack with port_dump().

Needed for #946

We can do it for free now as all functions used by call.cc have already
been converted to C and there's nothing in call.cc that really needs any
C++ features.

This patch adds ability to reload C procedures on the fly without
downtime. To achive that, Tarantool loads a new copy of shared
library and starts routing all new request to the new version.
The previous version remains active until all started calls
are finished. All shared libraries are loaded with RTLD_LOCAL,
therefore two or more copies can co-exist without any problems.
From now box loads all external modules via an unique symlink to
avoid caching inside dlopen().

If one of some module function is reloaded then all other functions
from this module will be reloaded.

Reviewed and heavily patched by Roman Tsisyk.

Closes #910

Needed for #910