HttpEventCollector in serial mode causes unnecessary Large Object Heap allocations

[graemefoster]

Due to when it realises the StringBuilder's ToString() call, under high load with larger batches, it can result in MB to GB of allocations of byte[] on the Large Object Heap.

I'm going to put up a Pull Request which can have large impact on Private Bytes used by a process that serially logs under load (with little to no impact on other use-cases).