can't calculate replication lag if "lower_case_table_names=1"

[csuzhangxc]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do? If possible, provide a recipe for reproducing the error.

   set lower_case_table_names=1 for the upstream MySQL.

   set enable-heartbeat: true for task.yaml.

2. What did you expect to see?

   valid application lag in Grafana dashboard.

3. What did you see instead?

   invalid replication lag in Grafana dashboard.

4. Versions of the cluster

   • DM version (run dmctl -V or dm-worker -V or dm-master -V):

     v1.0.5
     

[csuzhangxc]

in pingcap/tidb-tools#322, we renamed dm_heartbeat/heartbeat to DM_HEARTBEAT/HEARTBEAT.

but MySQL has lower_case_table_names, which will convert upper case to lower case.

and in DM, we use == to compare a table name whether is heartbeat's table name before calculate replication lag.