Remove Send and Sync from core crate

[devrandom]

Related to #842

[codecov]

Codecov Report

Merging #904 (ec35fe6) into main (cb0b4bf) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #904      +/-   ##
==========================================
+ Coverage   90.37%   90.39%   +0.01%     
==========================================
  Files          57       57              
  Lines       29500    29500              
==========================================
+ Hits        26662    26666       +4     
+ Misses       2838     2834       -4     

Impacted Files	Coverage Δ
lightning-block-sync/src/poll.rs	92.04% <ø> (ø)
lightning-net-tokio/src/lib.rs	76.42% <ø> (ø)
lightning/src/chain/chainmonitor.rs	95.52% <ø> (ø)
lightning/src/chain/channelmonitor.rs	95.45% <ø> (ø)
lightning/src/chain/keysinterface.rs	93.62% <ø> (ø)
lightning/src/chain/mod.rs	100.00% <ø> (ø)
lightning/src/ln/channelmanager.rs	83.42% <ø> (ø)
lightning/src/ln/msgs.rs	88.76% <ø> (ø)
lightning/src/routing/network_graph.rs	91.86% <ø> (ø)
lightning/src/util/logger.rs	57.14% <ø> (ø)
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cb0b4bf...ec35fe6. Read the comment docs.