Significant increase in total memory allocation in the request handler

[VladoLavor]

The request handler excessively uses logrus.WithFields() call to collect entries for the debug logger.

Afaik, each WithFields() call creates a new *logrus.Entry, duplicating the existing underlying map, which is very expensive, especially in a high-frequency code path like this. The built object is not even used if the debug logging is disabled.

These changes were introduced in PR#281

I noticed this when doing continuous memory benchmarks in the Frouter environment. Before the change was introduced, the total allocated memory for 1M requests was under 1GB

│                │    Measured    │    Threshold   │
├────────────────┼────────────────┼────────────────┤
│ Total alloc    │      901.94 MB │        1.50 GB 

In v0.12:

│                │    Measured    │    Threshold   │
├────────────────┼────────────────┼────────────────┤
│ Total alloc    │       11.14 GB │        1.50 GB │

I pushed a draft PR that shows a possible solution by building logrus entry only when needed (it contains a lot of duplicated code to take it as an example). The change at least ensures that the latest GoVPP meets the memory thresholds set in v0.11.