Vladislav Shpilevoy

Now when there are both `TCPSocket` and `SSLSocket` available in `mg::aio`, it should be possible to implement an `HTTPClient`. And an `HTTPServer` at least for testing the client.

It is a useful metric in general, to see how much pressure does WAL get. And would be amazing to have that for tests.

Synchronous Promote - Part 1 - Limbo State Refactoring

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This PR reworks the limbo in a way that its state control code is more focused and the limbo now has an explicit state.

replication: `healthy_count` is outdated / lagging

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`replicaset.healthy_count` seems to be updated incorrectly sometimes. Or too late. Here is a test: ```Lua -- -- Instance 1 -- -- Step 1 -- fiber = require('fiber') log = require('log')...

This is a generalization of #11807. That ticket was about a crash which was happening when `txn_limbo_is_empty()` is false, but `txn_limbo_last_synchro_entry()` is `NULL`. Which sounds strange, but it is theoretically...