Add mutex lock on write-to-file function

[bjornaer]

proposed solution for #4

as the issue states, the idea is adding a lock when block service writes to file

[naqvijafar91]

@bjornaer Looks good to me, but can you also write some integration tests with concurrent goroutines trying to write and then in the end verify that all the required data is finally written and nothing is corrupted. Bdw, thanks for showing interest, i created this so that i can understand the internals of how a database works, and this PR would make it more close to actual minimal use cases.

[bjornaer]

@naqvijafar91 definitely, will do so then