port_lua: allow to set bottom index

Since fiber lua state can be used for internal purposes, we cannot
truncate it, so we need an abitily to create port_lua with lower index
boundary. The patch populates the port with field bottom - dump will
start from this index at Lua stack.

Also, the patch modifies port_lua destructor - all unused values are
popped from lua stack now to prevent Lua stack overflow.

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