Change: remove N, LS, SM from Raft<C, N, LS, SM>

[drmingdrmer]

Changelog

Change: remove N, LS, SM from Raft<C, N, LS, SM>

• Raft<C, ..>: is a control handle of RaftCore and it does not directly
  rely on N: RaftNetworkFactory, LS: RaftLogStorage and
  SM: RaftStateMachine.
  Thus these types should not be part of Raft.

  In this commit, we remove N, LS, SM from Raft<C, N, LS, SM>,
  RaftInner<C, N, LS> and RaftMsg<C, N, LS>.
  Type N, LS, SM now are only used by Raft::new() which needs these
  types to create RaftCore.

• Raft::external_request(): Another change is the signature of the
  Fn passed to Raft::external_request() changes from
  FnOnce(&RaftState, &mut LS, &mut N) to FnOnce(&RaftState).

• Fix: the FnOnce passed to Raft::external_request() should always
  be Send, unoptionally. Because it is used to send from Raft to
  RaftCore.

• Fix: RaftMsg::ExternalRequest: should not rely on RaftLogStorage and RaftNetworkFactory #939

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This change is 

[schreter]

Reviewed 18 of 18 files at r1, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @ariesdevil, @drmingdrmer, and @lichuang)

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openraft/src/raft/mod.rs line 145 at r1 (raw file):

// Notably, the state machine, log storage and network factory DO NOT have to be `Send`, those
// are only used within Raft task(s) on a single thread.
unsafe impl<C> Send for Raft<C>

This can then be removed, since it's implicitly Send.

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openraft/src/raft/mod.rs line 160 at r1 (raw file):

//
// See above for details.
unsafe impl<C> Sync for Raft<C>

Same here.

[drmingdrmer]

This can then be removed, since it's implicitly Send.

Thank you! I missed this part.