These changes improves the performance of the non-coloured terminal formatting, _quite a lot_.
```
name old time/op new time/op delta
TerminalHandler-8 10.2µs ±15% 5.4µs ± 9% -47.02% (p=0.008 n=5+5)
name old alloc/op new alloc/op delta
TerminalHandler-8 2.17kB ± 0% 0.40kB ± 0% -81.46% (p=0.008 n=5+5)
name old allocs/op new allocs/op delta
TerminalHandler-8 33.0 ± 0% 5.0 ± 0% -84.85% (p=0.008 n=5+5)
```
I tried to _somewhat_ organize the commits, but the it might still be a bit chaotic. Some core insights:
- The function `terminalHandler.Handl` uses a mutex, and writes all output immediately to 'upstream'. Thus, it can reuse a scratch-buffer every time.
- This buffer can be propagated internally, making all the internal formatters either write directly to it,
- OR, make use of the `tmp := buf.AvailableBuffer()` in some cases, where a byte buffer "extra capacity" can be temporarily used.
- The `slog` package uses `Attr` by value. It makes sense to minimize operating on them, since iterating / collecting into a new slice, iterating again etc causes copy-on-heap. Better to operate on them only once.
- If we want to do padding, it's better to copy from a constant `space`-buffer than to invoke `bytes.Repeat` every single time.
This PR is a bit in preparation for the slog work in #28187 .
Our current test re logging mostly test the internals, but we have no real end-to-end test of the logging output. This PR introduces a simple reexec-based log tester. This also relies upon a special mode in geth, which can be made to eject a set of predefined log messages (only available if the build-tag `integrationtests` is used
e.g. go run --tags=integrationtests ./cmd/geth --log.format terminal logtest
While working on this, I also noticed a quirk in the setup: when geth was configured to use a file output, then two separate handlers were used (one handler for the file, one handler for the console). Using two separate handlers means that two formatters are used, thus the formatting of any/all records happened twice. This PR changes the mechanism to use two separate io.Writers instead, which is both more optimal and fixes a bug which occurs due to a global statefulness in the formatter.
* log: fix formatting of big.Int
The implementation of formatLogfmtBigInt had two issues: it crashed when
the number was actually large enough to hit the big integer case, and
modified the big.Int while formatting it.
* log: don't call FormatLogfmtInt64 for int16
* log: separate from decimals back, not front
Co-authored-by: Péter Szilágyi <peterke@gmail.com>
* all: add thousandths separators for big numbers on log messages
* p2p/sentry: drop accidental file
* common, log: add fast number formatter
* common, eth/protocols/snap: simplifty fancy num types
* log: handle nil big ints
