Keep requests under the ingestion size limit

[PetrHeinz]

A batch goes out as a single request however large it is, because the drain caps batches by count only (1000 logs). Since 1.1.0 sends the exception of every log, a burst of errors fills that easily: 1000 errors with a 12 KB stack trace each, logged within one flush period, make a 12.9 MB request. Ingestion answered that with 413 (it took 9.0 MB, the documented limit is 10 MB) and the whole batch was lost, exactly during an incident, when the backlog fills the batches.

• Client.Send serializes each log on its own and sends the batch as one or more requests, each a JSON array of at most 5 MB (one constant, with a comment naming the 10 MB ingestion limit). The order of the logs is kept, and every request goes through the retry logic on its own.
• A single log that does not fit in a 5 MB request is dropped with an error in NLog's internal log naming its size. The logs after it are delivered.
• Each log is serialized with the same settings as before, so the request body of a batch that fits in one request stays byte for byte the same. The new small-batch test pins such a body.
• Send(IEnumerable<Log>) keeps its signature. It no longer posts an empty [] when it is given no logs (the drain never does that).

The first commit adds the tests and is expected to be red: the 1000 logs arrive as one request of 12,764,828 bytes, and the 6 MB log is sent along with the logs around it. The small-batch test passes on both commits, it pins that the payload does not change. The second commit changes Client.Send.

The tests sit right before SplitsLogsIntoBatches instead of after it, because #17 adds its test right after it.

#25 and #26 both change Client.serialize and conflict there. Whichever merges second serializes each log through #26's depth-limiting writer, and lets each log nest one level less than the limit (Top >= MaxDepth - 1), since the log then goes into the array of the request. CutsOffPropertyNestedTooDeeply catches a missed adjustment: the test's JSON reader rejects a request nested deeper than 64 levels. A throwaway merge of all these branches with that resolution is green on NLog 4.7.11 and 6.*.

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