[Merged by Bors] - Correct bug in sync

[AgeManning]

A bug that causes failed batches to continually download in a loop is corrected.

[michaelsproul]

Let's merge this.

I'm also working on a Clippy lint so we can prevent it happening in future

[AgeManning]

bors r+

[bors]

Build failed:

• eth1-simulator-ubuntu

[michaelsproul]

bors retry

[michaelsproul]

Got my lint working 😎

error: casting function pointer `B::max_batch_download_attempts` to `usize`
   --> beacon_node/network/src/sync/range_sync/batch.rs:260:24
    |
260 |                     >= B::max_batch_download_attempts as usize
    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `B::max_batch_download_attempts() as usize`
    |
    = note: requested on the command line with `-D clippy::fn-to-numeric-cast-any`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#fn_to_numeric_cast_any

Clippy PR here: rust-lang/rust-clippy#7705

[bors]

Pull request successfully merged into unstable.

Build succeeded:

• arbitrary-check
• cargo-audit
• cargo-fmt
• cargo-udeps
• check-benchmarks
• check-consensus
• clippy
• debug-tests-ubuntu
• dockerfile-ubuntu
• ef-tests-ubuntu
• eth1-simulator-ubuntu
• no-eth1-simulator-ubuntu
• release-tests-ubuntu
• release-tests-windows
• state-transition-vectors-ubuntu