Rename EVT cache hash to make context name unique

Rename EVT cache hash to make context name unique

BuildEventTriggerCache sets up a context "EventTriggerCache" which
house a hash named "Event Trigger Cache", which in turn creates a
context with the table name.  This generated log output for memory
context dumps like below:

LOG:  level: 2; EventTriggerCache: 8192 total in 1 blocks; 7928 free (4 chunks); 264 used
LOG:  level: 3; Event Trigger Cache: 8192 total in 1 blocks; 2616 free (0 chunks); 5576 used

This renames the hash to ensure that the hash context has a unique
name for easier log reading and debugging.

Discussion: https://postgr.es/m/5EDC969E-CAE3-4CBD-965E-3B8A1294CFA4@yesql.se