Feature: add 'singlethreaded' raft mode

[wvwwvwwv]

• 'singlethreaded' compile-time feature gate.

The new feature gate forces the raft instance to be used by a single thread by not implementing Send for certain data structures and substituting calls to tokio::spawn with calls to tokio::spawn_local when using the tokio asynchronous runtime. In case the application wants to use a different type of asynchronous runtimes, the spawn method has to be appropriately implemented.

• Additional change included.

openraft::Instant is implemented for std::time::Instant to simplify the usage.

• Fix: #862

FYI: @schreter

Checklist

• Updated guide with pertinent info (may not always apply).
• Squash down commits to one or two logical commits which clearly describe the work you've done.
• Unittest is a friend:)

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This change is 

[drmingdrmer]

Neat. Thank you for your contribution on this issue.

Reviewed 21 of 21 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @schreter)

[schreter]

Yes, looks very good!

Reviewed 21 of 21 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @wvwwvwwv)

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macros/src/lib.rs line 11 at r1 (raw file):

/// in the same thread.
#[proc_macro_attribute]
pub fn add_async_trait(_attr: TokenStream, item: TokenStream) -> TokenStream {

BTW, should this macro be re-exported from openraft crate, so the user would then also use #[add_async_trait]?

[wvwwvwwv]

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @wvwwvwwv)

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macros/src/lib.rs line 11 at r1 (raw file):

Previously, schreter wrote…

BTW, should this macro be re-exported from openraft crate, so the user would then also use #[add_async_trait]?

Good idea!

[wvwwvwwv]

@drmingdrmer uploaded a new commit, but the change is very small :-)

Reviewable status: 19 of 21 files reviewed, all discussions resolved (waiting on @drmingdrmer and @schreter)

[drmingdrmer]

Reviewed 2 of 2 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @wvwwvwwv)