Persistent rules are lost due to improper use of memory id generator

## Issue Description

When I use external datasource to persist rules，Once SentinelDashboard restart，new rules will overwrite old rules when inserted, causing the old rules to be lost.

### Describe what happened (or what feature you want)
The rule ID generator currently in use is an AtomicLong，when dashboard restart，the AtomicLong will clean，However, the in-memory rule cache is loaded from the external datasource, and the rule ids start at 0. Insert a new rule at this point will override the old rule.

The code directory is：sentinel-dashboard/src/main/java/com/alibaba/csp/sentinel/dashboard/repository/rule/

### Describe what you expected to happen
Solving this problem requires a continuously increasing ID generator，For convenience, I used a timestamp instead，but the display order of rules in the Dashboard is inconsistent with the insert order

### How to reproduce it (as minimally and precisely as possible)

1. Configure an external data source to hold rule data
2. Insert a few rules
3. Restart sentinelDashboard and insert a few more rules

### environment
jdk 1.8

Start in sentinel-dashboard/src/main/java/com/alibaba/csp/sentinel/dashboard/repository/rule/ and inspect how rule IDs are generated, cached, and loaded from an external datasource. Reproduce the sequence of inserting rules, restarting SentinelDashboard, and inserting more rules; done means existing rules are not overwritten and newly inserted rules retain a consistent order.

Written by the indexing model from the issue text.