Use MEDIUMTEXT(16MB) type when Mysql is used

db/migrate/20180326202229_create_ci_job_trace_chunks.rb

@@ -8,7 +8,9 @@ class CreateCiJobTraceChunks < ActiveRecord::Migration
       t.integer :job_id, null: false
       t.integer :chunk_index, null: false
       t.integer :data_store, null: false
-      t.text :raw_data
+      # Mysql needs MEDIUMTEXT type (up to 16MB) rather than TEXT (up to 64KB)
+      # Because 'raw_data' is always capped by Ci::JobTraceChunk::CHUNK_SIZE, which is 128KB
+      t.text :raw_data, limit: 16.megabytes - 1
 
       t.foreign_key :ci_builds, column: :job_id, on_delete: :cascade
       t.index [:job_id, :chunk_index], unique: true