Release 7.6.0: Add user ID column for directoryChecksums table

[Guest]

To support stored directory checksums for folders where different users may get a different list of contained files, e.g. caused by custom permissions, a new column is added to store the user ID along with each directory checksum. The new CREATE TABLE statement now looks like:

CREATE TABLE directoryChecksums (
uuid BINARY(16) NOT NULL,
cid INT4 UNSIGNED NOT NULL,
user INT4 UNSIGNED,
folder VARCHAR(512) NOT NULL,
sequence BIGINT(20) NOT NULL,
checksum BINARY(16) NOT NULL,
PRIMARY KEY (cid, uuid),
INDEX (folder, cid),
INDEX (checksum, cid)
) ENGINE=InnoDB DEFAULT CHARSET=ascii;

Existing checksums are removed beforehand, as only user-specific checksums are used in the future.