update docs for refresh_interval setting
ensure that a _version=0 query has no match

and does not execute
Merge pull request #873 from crate/m/refresh-docs

update docs for refresh_interval setting
update docs about group by and aggregates
Merge pull request #874 from crate/m/group_by_docs

update docs about group by and aggregates
change substring function to handle null values correctly
implement "alter table add column"
refactor create table statement analyzer

in order to

 * share code between create table and alter table add column
 * avoid empty lists in the mapping which break merging of the mappings