Transaction and cursor server pinning support added

[taganaka]

Hey there!

This PR aims to implement 2 known missing features:

• Transaction server pinning
• Different cursors on separate servers with the same cursor ID value

Cursors are pinned to a server selection by using <cursorID>-<collection-name> as unique identifier (could't find better identifier)

Transactions pinning use lsid and that should be random enough to guarantee uniqueness among different connections