perf: mesgdef micro optimisation

[muktihari]

• sync.Pool now use max fields in the entire messages instead of 255. In the current Profilx.xlsx, the maximum fields is 156 (from session message), reducing approx. 99 * unsafe.Sizeof(Field{}) memory usage per created pool object.
• ToMesg now compare the invalid value of time without converting it to uint32 first. If the value valid, convert the value directly without using datetime.ToUint32 since under the hood it will do comparison.

Benchmark:

goos: darwin
goarch: amd64
pkg: github.com/muktihari/fit/profile/mesgdef
cpu: Intel(R) Core(TM) i5-5257U CPU @ 2.70GHz
               │  old.txt   │              new.txt               │
               │   sec/op    │   sec/op     vs base                │
RecordToMesg-4   214.1n ± 1%   174.6n ± 2%  -18.43% (p=0.000 n=10)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (091ed76) to head (9d5b4a4).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #363 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 41 41 Lines 3663 3662 -1 ========================================= - Hits 3663 3662 -1 ```

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