httpc: replace ibuf_alloc with xibuf_alloc There is no check for NULL for a value returned by `ibuf_alloc`, the NULL will be passed to `memcpy()` if the aforementioned function will return a NULL. The patch fixes that by replacing `ibuf_alloc` with macros `xibuf_alloc` that never return NULL. Found by Svace. NO_CHANGELOG=codehealth NO_DOC=codehealth NO_TEST=codehealth (cherry picked from commit b4ee146fde6e418aed590ac6054cff75c2a59626)

limbo: speed up synchronous transaction queue processing This patch optimizes the process of collecting ACKs from replicas for synchronous transactions. Before this patch, collecting confirmations was slow in some cases. There was a possible situation where it was necessary to go through the entire limbo again every time the next ACK was received from the replica. This was especially noticeable in the case of a large number of parallel synchronous requests. For example, in the 1mops_write bench with parameters --fibers=6000 --ops=1000000 --transaction=1, performance increases by 13-18 times on small clusters of 2-4 nodes and 2 times on large clusters of 31 nodes. Closes #9917 NO_DOC=performance improvement NO_TEST=performance improvement (cherry picked from commit 4a866f64d64c610a3c8441835fee3d8dda5eca71)

vclock: introduce `vclock_nth_element` and `vclock_count_ge` Two new vclock methods have been added: `vclock_nth_element` and `vclock_count_ge`. * `vclock_nth_element` takes n and returns whatever element would occur in nth position if vclock were sorted. This method is very useful for synchronous replication because it can be used to find out the lsn of the last confirmed transaction - it's simply the result of calling this method with argument {vclock_size - replication_synchro_quorum} (provided that vclock_size >= replication synchro quorum, otherwise it is obvious that no transaction has yet been confirmed). * `vclock_count_ge` takes lsn and returns the number of components whose value is greater than or equal to lsn. This can be useful to understand how many replicas have already received a transaction with a given lsn. Part of #9917 NO_CHANGELOG=Will be added in another commit NO_DOC=internal (cherry picked from commit 58f3c93b660499e85f08a4f63373040bcae28732)

memtx: do not pass NULL to memcpy when creating gap item in MVCC According to the C standard, passing `NULL` to `memcpy` is UB, even if it copies nothing (number of bytes to copy is 0). The commit fixes such situation in memtx MVCC. Closes tarantool/security#129 NO_TEST=fix UB NO_CHANGELOG=fix UB NO_DOC=fix UB

net_box: fix a crash when a trigger deletes itself In 6d88274 we rewrote Lua module `internal.trigger` that is used in `net.box` and possibly some external modules. Old implementation held all triggers in a Lua table, so deleting a trigger on the fly wasn't a problem. Now we store all triggers in a list and simply iterate over it when the triggers are fired, so the trigger list became non-resistent to modifications on the fly. In order to fix it, let's simply use `trigger_run` - it is resistant to the list modifications. Closes #10622 NO_DOC=bugfix (cherry picked from commit 78cfc5ef4cf43bf9d53dc51d0f843bee350609fd)