iproto: protect from false-correct size in msg header

Consider this packet:

    msgpack = require('msgpack')
    data = msgpack.encode(18400000000000000000)..'aaaaaaa'

Tarantool interprets 18400000000000000000 as size of a coming
iproto request, and tries with no any checks to allocate buffer
of such size. It calculates needed capacity like this:

    capacity = start_value;
    while (capacity < size)
        capacity *= 2;

Here it is possible that on i-th iteration 'capacity' < 'size',
but 'capacity * 2' overflows 64 bits and becomes < 'size' again,
so this loop never ends and occupies 100% CPU.

Strictly speaking overflow has undefined behavior. On the
original system it led to nullifying 'capacity'.

Such size is improbable as a real packet gabarits, but can appear
as a result of parsing of some invalid packet, first bytes of
which accidentally appears to be valid MessagePack uint. This is
how the bug emerged on the real system.

Lets restrict the maximal packet size to 2GB.

Closes #3464