WL#7270: PERFORMANCE SCHEMA, configurable statement text size

Currently in the performance schema, the statement instrumentation is limited as
follows:

1)
Column SQLTEXT, for a statement text, is truncated:
#define COL_INFO_SIZE 1024

2)
Column DIGEST_TEXT, for a statement digest text, is truncated:
#define COL_DIGEST_TEXT_SIZE 1024 (for the expanded text)
#define PSI_MAX_DIGEST_STORAGE_SIZE 1024 (for the storage format)

These limits are set at build time, and not configurable.

It can be desirable to change these limits, at server startup:
- to lower values to save memory
- to higher values to get better resolution, for applications using very long
(typically generated) statements.