Fix bug in get_block of chain_plugin that would lead to unnecessary failures for some block numbers

[arhag]

Change Description

This PR fixes a bug in get_block of chain_plugin that could lead to unnecessary failure when requesting a block by number. This PR also updates the fc submodule to the latest master.

The get_block logic first tried to convert the block_num_or_id string input into a block ID and use it to try to find the block. If it failed to find the block, it would then attempt interpret the string as a block number and try to find the block that way.

But the request should not just fail if the string input does not make a valid ID when it could make a perfectly valid block number. This was the failure mode that was being triggered in one of the tests after the fc submodule update. EOSIO/fc#64 added stricter validation rules to conversions from fc::variant to vector<char>, which fc::variant conversions to block_id_type uses, to disallow odd-length hex strings. So with these fc changes, the original logic of get_block would fail requests made with block numbers that when represented as a decimal string were of odd-length.

The new logic is aligned with how get_block_header_state already worked. It will try to interpret block_num_or_id as a number. If it can, it will try to lookup the block by the number. If it cannot interpret block_num_or_id as a number, it will then try to convert the string to a block ID (failing the request if not possible) and then use that ID to try to lookup the block.

Consensus Changes

API Changes

Documentation Additions