Rethink the wait event names for postgres_fdw, dblink and etc

Hi,

Recently, the API to define custom wait events for extension is 
supported.
* Change custom wait events to use dynamic shared hash tables(af720b4c5)

So, I'd like to rethink the wait event names for modules which use
"WAIT_EVENT_EXTENSION" wait events.
* postgres_fdw
* dblink
* pg_prewarm
* test_shm_mq
* worker_spi

I expect that no one will object to changing the names to appropriate
ones. But, we need to discuss that naming convention, the names 
themselves,
document descriptions and so on.

I made the v1 patch
* CamelCase naming convention
* Add document descriptions for each module

I haven't added document descriptions for pg_prewarm and test modules.
The reason is that the wait event of autoprewarm is not shown on
pg_stat_activity. It's not an auxiliary-process and doesn't connect to
a database, so pgstat_bestart() isn't be called.

Feedback is always welcome and appreciated.

Regards,
-- 
Masahiro Ikeda
NTT DATA CORPORATION