Add the possibility to connect to a specific database

[andrejtonev]

Add multi-tenant capability.

conn = mgclient.connect(address...., database="db")

will return a connection that is limited to the defined database.

[andrejtonev]

@kgolubic We have added the ability to define the landing database via pymgclient.
mgclient.connect(database="your db", ...)
The connection is then locked to the defined db, you cannot switch to another one later on.

[andrejtonev]

FYI @Josipmrden this PR requires a fix in memgraph.
Should be delivered via memgraph/memgraph#1884