box: reuse formats exported to Lua

Make lbox_tuple_format_new (exported as box.internal.new_tuple_format to
Lua) reuse tuple formats. It's safe, because formats exported to Lua are
immutable. This fixes a net.box error when one creates/closes a lot of
net.box connections: a net.box connection creates a few formats for a
local version of the schema so if the garbage collector isn't invoked to
clean up after it, Tarantool can run out of format ids.

Closes #6217