Remove snapshots and unify them with shape log instead

[kevin-dp]

Currently, Electric stores an initial snapshot and a shape log for every shape. However, the snapshot is a sequence of inserts and thus can be stored in the shape log, no need for a separate snapshot.

For example, assume the following initial snapshot: [ {op: "insert", row: <row1>}, ..., {op: "insert", row: <row g>} ]

And shape log: [ {op: "insert", row: <row h>}, ..., {op: "insert", row: <row j>} ]

We can remove the snapshot and unify it with the shape log: [ {op: "insert", row: <row1>}, ..., {op: "insert", row: <row g>}, {op: "insert", row: <row h>}, ..., {op: "insert", row: <row j>} ]