rename ReloadPolicy onCommit to onCommitWithDelay

[giovannicuccu]

Here is the pull request for issue #1824 that we dicussed on discord.
I simply renamed the reload policy from OnCommit to OnCommitWithDelay and updated the reference in the documentation and tests.

I tried to implement an OnCommit policy for synchronous reload but I did not succeed.
Here are the main issues I faced:

1. IndexReader and IndexWriter can be created indipendently (see test_index_on_commit_reload_policy_different_directories in core/test.rs ) and as far as I understand they share the access to the same directory. When an IndexWriter commits the changes are propagated to the file system, if an IndexReader is executed in a different process there is no simple way to notify the commit in a synchronous way because "there isn't a channel" between them (aside from the file system). I checked if there are some notifications for updating mmapped files but I did not find anything
2. There semantic of the reload is "lost" as the callback is passed to the directory object that implements the commit notification in a different way (worker thread for mmap_directory, direct call inside atomic_write for ram_directory). If the ReloadPolicy is "directory implementation indipendent" there is the need to associate it to the callback.

If you think it'd feasible to implement a sync OnCommit policy I ask for some advice and I'll start to dig further into the problem.

[PSeitz]

Thanks! As of now you can implement sync behavior by manually reloading, which should be fine for most use cases