Support escape sequence for cluster_name in postgres_fdw.application_name

Hi,

Commit 6e0cb3dec1 allowed postgres_fdw.application_name to include escape sequences %a (application name), %d (database
name),%u (user name) and %p (pid). In addition to them, I'd like to support the escape sequence (e.g., %C) for cluster
namethere. This escape sequence is helpful to investigate where each remote transactions came from. Thought?
 

Patch attached.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION