Zano build v1.1.6.87 released

Changes:

• Blockchain pre-downloading on a poor connection greatly improved
• GUI: default mixin count set to 10
• core: added checkpoint for height 525000 (mainnet) and height 350000 (testnet)
• core: pre-downloading blockchain DB updated (for both engines: lmdb and mdbx)
• minor bugfixes and improvements

Binaries and sources: https://github.com/hyle-team/zano/releases/tag/1.1.6.87