feat: added interval based wal synchronization

[hitesh22rana]

Feature: #13 (comment)
Asynchronously sync the data to disk at configurable time interval.

[hitesh22rana]

There seems to be some failed tests.
Let me take a look and revert to you 🙇🏼

[hitesh22rana]

@roseduan PTAL 🙏🏼,
Tests seems to be fixed now.

Reasons of test failure in first place
since, we are already closing wal via destroyWAL function in wal_test.go, in some of the test cases it was also trying to invoke the Close() explicitly which was causing panic.

Explicit check is now added in Close() function in wal.go to only close the channel if it is not already close.

[hitesh22rana]

Hi @roseduan, just a friendly reminder to PTAL when you get a chance 🙏🏼.

The tests seem stable now, and I've addressed the root cause of the previous failures (detailed above). Let me know if there's anything else you'd like me to adjust or clarify.
Thank you!

[roseduan]

LGTM