Fix reload callback session construction + remove duplication of code by refactoring the callback/promise handling code

[oom-]

Tests:

 PASS ​ test/memorystore.test.js 16 OK 63.406ms    
​ PASS ​ test/idGenerator.test.js 512 OK 140.972ms
​ PASS ​ test/fastifySession.checkOptions.test.js 12 OK 146.505ms
​ PASS ​ test/store.test.js 11 OK 189.105ms
​ PASS ​ test/base.test.js 62 OK 460.55ms
​ PASS ​ test/cookie.test.js 72 OK 508.147ms
​ PASS ​ test/session.test.js 123 OK 677.57ms
​ PASS ​ test/verifyPath.test.js 34 OK 116.583ms
​ PASS ​ test/expiration.test.js 5 OK 2s



  🌈 SUMMARY RESULTS 🌈  


Suites:   ​9 passed​, ​9 of 9 completed​
Asserts:  ​​​847 passed​, ​of 847​
​Time:​   ​4s
-------------------|---------|----------|---------|---------|-------------------
File               | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
-------------------|---------|----------|---------|---------|-------------------
All files          |     100 |      100 |     100 |     100 |
 cookie.js         |     100 |      100 |     100 |     100 |
 fastifySession.js |     100 |      100 |     100 |     100 |
 idGenerator.js    |     100 |      100 |     100 |     100 |
 session.js        |     100 |      100 |     100 |     100 |
 store.js          |     100 |      100 |     100 |     100 |
-------------------|---------|----------|---------|---------|-------------------

Benchmark (they do not run on my machine even with clean install):

╔══════════════╤═════════╤═════════╤═══════════╤═════════════════════════╗
║ Slower tests │ Samples │  Result │ Tolerance │ Difference with slowest ║
╟──────────────┼─────────┼─────────┼───────────┼─────────────────────────╢
║ redis        │       0 │ Errored │       N/A │ N/A                     ║
║ file         │       0 │ Errored │       N/A │ N/A                     ║
╟──────────────┼─────────┼─────────┼───────────┼─────────────────────────╢
║ Fastest test │ Samples │  Result │ Tolerance │ Difference with slowest ║
╟──────────────┼─────────┼─────────┼───────────┼─────────────────────────╢
║ memory       │       0 │ Errored │       N/A │ N/A                     ║
╚══════════════╧═════════╧═════════╧═══════════╧═════════════════════════╝

Checklist

• [x] run npm run test and npm run benchmark
• [x] tests and/or benchmarks are included
• [x] documentation is changed or added (no)
• [x] commit message and code follows the Developer's Certification of Origin and the Code of conduct