We will use this method in upcoming commit to create
client from stored bookmark.

Given a bookmark path and bookmark name, GetBookmark
returns a Bookmark object that corresponds to the
stored bookmark settings.

In next commits, we will use this method to read
stored bookmarks and create a db client.

We know that a port is a number. Lets enforce that
rule at type level by setting it so.

This commit also adjusts test funcs and helper data
to fit Port's new int type.

We will use client.NewFromURL to create a client
from stored bookmark in next commit. Since
client.NewFromURL takes in a command.Options, lets
add a method on Bookmark to get corresponding Option.

if options.Bookmark is set, initClient will now use
it to create a client from the bookmarked settings.

initClientUsingBookmark uses methods introduced in
previous commits. It constructs connection string
to db from Bookmark.Url if it exists. If not, it
uses other fields in Bookmark to construct the
connection string. An implication of this is that
the Url field of Bookmark takes precedence.

The codebase doesn't yet use the explicitly typed error pattern. To keep
things consistent, lets use the generic error type.

ConvertToOptions can't return any error.