Improve OCC applied to high contention storage writes.

The format is based on [keep a changelog](http://keepachangelog.com) and this project uses [semantic versioning](http://semver.org).

## [Unreleased]

### Added

- Custom events API for client and runtime events.

### Changed

- Default runtime HTTP key value is no longer the same as the default Server key value.

- Group create now returns HTTP 409 Conflict/GRPC Code 6 when group name is already in use.

### Fixed

- Correctly handle errors when concurrently writing new storage objects.

- Correctly apply optimistic concurrency controls to individual storage objects under high write contention.

## [2.7.0] - 2019-09-11

### Added

- Enable RPC functions to receive and return raw JSON data.

	"encoding/gob"

	"errors"

	"fmt"

	"github.com/jackc/pgx"

	"sort"

	"context"

		return ack, nil

	}

	params := []interface{}{object.Collection, object.Key, ownerID, object.Value, newVersion, newPermissionRead, newPermissionWrite}

	var query string

	if dbVersion.Valid {

		// Updating an existing storage object.

		query = "UPDATE storage SET value = $4, version = $5, read = $6, write = $7, update_time = now() WHERE collection = $1 AND key = $2 AND user_id = $3::UUID"

	} else {

		// Inserting a new storage object.

	switch {

	case object.Version != "" && object.Version != "*":

		// OCC if match.

		query = "UPDATE storage SET value = $4, version = $5, read = $6, write = $7, update_time = now() WHERE collection = $1 AND key = $2 AND user_id = $3::UUID AND version = $8"

		params = append(params, object.Version)

		// Respect permissions in non-authoritative writes.

		if !authoritativeWrite {

			query += " AND write = 1"

		}

	case dbVersion.Valid && object.Version != "*":

		// An existing storage object was present, but no OCC if-not-exists required.

		query = "UPDATE storage SET value = $4, version = $5, read = $6, write = $7, update_time = now() WHERE collection = $1 AND key = $2 AND user_id = $3::UUID AND version = $8"

		params = append(params, dbVersion.String)

		// Respect permissions in non-authoritative writes.

		if !authoritativeWrite {

			query += " AND write = 1"

		}

	default:

		// OCC if-not-exists, and all other non-OCC cases.

		query = "INSERT INTO storage (collection, key, user_id, value, version, read, write, create_time, update_time) VALUES ($1, $2, $3::UUID, $4, $5, $6, $7, now(), now())"

		// Existing permission checks are not applicable for new storage objects.

	}

	res, err := tx.ExecContext(ctx, query, object.Collection, object.Key, ownerID, object.Value, newVersion, newPermissionRead, newPermissionWrite)

	res, err := tx.ExecContext(ctx, query, params...)

	if err != nil {

		logger.Debug("Could not write storage object, exec error.", zap.Any("object", object), zap.String("query", query), zap.Error(err))

		if e, ok := err.(pgx.PgError); ok && e.Code == dbErrorUniqueViolation {

			return nil, ErrStorageRejectedVersion

		}

		return nil, err

	}

	if rowsAffected, _ := res.RowsAffected(); rowsAffected != 1 {

		logger.Debug("Could not write storage object, rowsAffected error.", zap.Any("object", object), zap.String("query", query), zap.Error(err))

		return nil, ErrStorageWriteFailed

		return nil, ErrStorageRejectedVersion

	}

	ack := &api.StorageObjectAck{