memtx: make stable DDL operations in mvcc mode

[Korablev77]

Now it is technically possible to execute DDL requests in MVCC mode. Nevertheless TX manager hasn't been adapted to DDL yet. As a result we can observe many bugs involving DDL operations: #5998 #6137 #6007

One of the problems is that caches for metadata structures (like space_cache, user_cache, func_cache) are not versioned. Possible solution is to keep lists of corresponding structures with the TX id for each unique entry in the cache. It is likely to be enough for the most structures (user, func, coll).

More examples:

-- Can't create spaces with the same name in separate transactions, meanwhile
-- drop doesn't find the space by name.
--
box.cfg{memtx_use_mvcc_engine = true}

txn_proxy = require('txn_proxy')

tx1 = txn_proxy.new()
tx2 = txn_proxy.new()

tx1:begin()
tx2:begin()

tx1("box.schema.create_space('t')")
tx2("box.schema.create_space('t')")
-- 'Space ''t'' already exists'

tx1:begin()
tx2:begin()


tx1("box.schema.create_space('t1')")
tx2("box.space.t1:drop()")
-- 'Space ''t1'' does not exist'

[Korablev77]

Here's example from #5515:

fiber = require('fiber')
channel1 = fiber.channel()
channel2 = fiber.channel()
function test1()                                 \
    box.begin()                                  \
    local s = box.schema.space.create('test')    \
    s:create_index('pk')                         \
    channel1:put(1)                              \
    local ok = channel2:get()                    \
    box.session.push(ok)                         \
    box.rollback()                               \
    channel1:put(1)                              \
end
f = fiber.new(test1)
channel1:get()
tx1('box.space.test:select()')
tx1('box.space.test:replace({1, 2, 3})')

tx1 sees space created in test1() meanwhile it shouldn't. In case we prohibit fiber yields between end of alter and TX preparation, this example will become impossible.