Add Ctx for cancellation to BlockPersistence.WriteNextBlock()

[ronnno]

and possibly other adapter methods.
all IO bound operations must be cancellable

[vistra]

This may be a bit tricky as file operations don't take context.
This may be achievable using write timeouts and retries, or using low level methods (e.g. closing a file descriptor from another thread).

Also - need to make sure that if a cancellation happens in the middle of a sequence of write operations, the file is not remained corrupted due to partial writes.

See related proposal, which also suggests alternative solutions - golang/go#20280

[ronnno]

The current filesystem adapter withstands interrupted writes. no need to guarantee a write has completed once began. the reader ignores partial or invalid entries and assumes the file ended in the last valid block.

However, it is a challenge to interrupt file operations..