Add boost/asio/io_context.hpp header to transaction_metadata.hpp for branch release/1.8.x

[johndebord]

Prior to this change it is possible for the build to fail with the following dependency error:

In file included from /Users/john.debord/eosio/eos-develop/libraries/chain/resource_limits.cpp:4:
/Users/john.debord/eosio/eos-develop/libraries/chain/include/eosio/chain/transaction_metadata.hpp:63:90: error: no type named 'io_context' in namespace 'boost::asio'
      static void start_recover_keys( const transaction_metadata_ptr& mtrx, boost::asio::io_context& thread_pool,
                                                                            ~~~~~~~~~~~~~^

Now this is not so.