Migrate to Serilog PeriodicBatching and increase throughput

[kendallmiller]

We were running into a performance issue and when researching determined that the Serilog.Sinks.PeriodicBatching provider was a better fit for Azure Log Analytics. The overwhelming changes were to normalize the configuration with Serilog's PeriodBatching configuration.

Of note:

• EmitBatchAsync replaces WriteLogEventAsync. EmitBatchAsync is provided by and called by Serilog.
• Changes to PostDataAsync remove the need for the semaphore - it no longer modifies default properties of the client and the list it is passed is not modified by EmitBatchAsync.
• Calls to PostDataAsync are independently asynchronous and can overlap for the same batch (so sending large batches is much faster).