getSelectTimestampMillisSql implementation for H2 SqlVendor
Revert "getSelectTimestampMillisSql implementation for H2 SqlVendor"

This reverts commit 7516082535c00b57658f694e4a0671ba93a3b3e3.
Changes SqlDatabase#now() to use an offset of 0 for SqlVendors that do not support it.
Changes PredicateParser#evaluate to return true when the given predicate is null to prevent NPEs.
Simplifies access to run Metric Resummarization from /_debug/code.

Improves error handling.
Improves Metric Resummarization's DistinctIdsIterator to avoid
LIMIT/OFFSET which was causing timeouts on large datasets.
Merge branch 'release/2.3'

Conflicts:
	db/src/main/java/com/psddev/dari/db/MetricResummarizationTask.java