program: Relax minimum rent check

[febo]

Problem

Currently, Allocate, Extend, Initialize and Write include explicit minimum rent exempt, but could rely on the runtime instead.

Solution

For Allocate and Initialize, replace the minimum rent check by a check that account must have lamports; these are the only 2 instructions that can create the PDA. Remove the rent exempt check in Extend and Write since these instructions require an initialized account with data.