fix(file source): sleep to avoid split reads

[lukesteensen]

Closes #1125

The file watcher works by repeatedly reading from the file being tailed until it sees a newline or EOF (i.e. the read syscall returns 0). Whatever data it has accumulated to that point is returned as a line.

When reads are closely interleaved with writes, however, it's possible for read to return part of a single write, and then EOF, and then the rest of that same write sometime later. This causes the file watcher to split what should be a single line into two. We've seen this happen ~3 times over 30M+ reads in our test environment.

This change causes the file watcher to wait 1 ms for any in-flight writes to become fully visible before returning a line that was never actually terminated with a newline.

[binarylogic]

@Jeffail do you mind reviewing this? Thank you sir.