libs/log: use fmt.Fprintf directly with *bytes.Buffer to avoid unnecessary allocations (#6503)

Noticed from profiling that the loggers are memory hungry and alas,
we were passing fmt.Sprintf strings to (*bytes.Buffer).WriteString
which defeats the purpose of using fmt.* This change fixes that and
instead directly invokes fmt.Fprintf.
The benchmarks show the improvement:

```shell
$ benchstat before.txt after.txt
name                     old time/op    new time/op    delta
TMLoggerSimple-8           1.67µs ± 4%    1.69µs ±13%     ~     (p=0.118 n=20+19)
TMLoggerContextual-8       2.01µs ± 8%    1.94µs ± 1%   -3.79%  (p=0.000 n=18+20)
TMFmtLoggerSimple-8        1.20µs ± 3%    1.16µs ± 2%   -3.39%  (p=0.000 n=20+16)
TMFmtLoggerContextual-8    1.53µs ±19%    1.43µs ±13%   -6.10%  (p=0.001 n=20+20)

name                     old alloc/op   new alloc/op   delta
TMLoggerSimple-8             696B ± 0%      616B ± 0%  -11.49%  (p=0.000 n=20+20)
TMLoggerContextual-8       1.02kB ± 0%    0.94kB ± 0%   -7.87%  (p=0.000 n=20+20)
TMFmtLoggerSimple-8          240B ± 0%      160B ± 0%  -33.33%  (p=0.000 n=20+20)
TMFmtLoggerContextual-8      416B ± 0%      336B ± 0%  -19.23%  (p=0.000 n=20+20)

name                     old allocs/op  new allocs/op  delta
TMLoggerSimple-8             13.0 ± 0%      12.0 ± 0%   -7.69%  (p=0.000 n=20+20)
TMLoggerContextual-8         17.0 ± 0%      16.0 ± 0%   -5.88%  (p=0.000 n=20+20)
TMFmtLoggerSimple-8          6.00 ± 0%      5.00 ± 0%  -16.67%  (p=0.000 n=20+20)
TMFmtLoggerContextual-8      8.00 ± 0%      7.00 ± 0%  -12.50%  (p=0.000 n=20+20)
```

Fixes #6502

libs/log/tmfmt_logger.go

@@ -98,7 +98,7 @@ func (l tmfmtLogger) Log(keyvals ...interface{}) error {
 	//     D										- first character of the level, uppercase (ASCII only)
 	//     [2016-05-02|11:06:44.322]    - our time format (see https://golang.org/src/time/format.go)
 	//     Stopping ...					- message
-	enc.buf.WriteString(fmt.Sprintf("%c[%s] %-44s ", lvl[0]-32, time.Now().Format("2006-01-02|15:04:05.000"), msg))
+	fmt.Fprintf(&enc.buf, "%c[%s] %-44s ", lvl[0]-32, time.Now().Format("2006-01-02|15:04:05.000"), msg)
 
 	if module != unknown {
 		enc.buf.WriteString("module=" + module + " ")