Fix transaction() not actually using a transaction

[Octogonapus]

transaction() used to just turn off autocommit, which means that rollback doesn't work:

julia> transaction(db) do
           stmt = prepare(db, "insert into foo (a) values (?);")
           execute(stmt, [6])
           error("force rollback")
       end
ERROR: force rollback
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] (::var"#1#2")()
   @ Main ./REPL[6]:4
 [3] transaction(f::var"#1#2", ::MySQL.Connection)
   @ DBInterface ~/.julia/packages/DBInterface/1Gmxx/src/DBInterface.jl:166
 [4] top-level scope
   @ REPL[6]:1

julia> DataFrame(execute(db, "select * from foo"))
6×1 DataFrame
 Row │ a      
     │ Int32? 
─────┼────────
   1 │      1
   2 │      1
   3 │      1
   4 │      2
   5 │      1
   6 │      6

I didn't find any C API for starting a transaction, so I am using the SQL instead. ref. https://dev.mysql.com/doc/c-api/8.0/en/c-api-basic-function-reference.html