Simplify savepoint ID logic
Simplify savepoint ID logic

A savepoint ID is now the "depth" of the savepoint.

sfackler/rust-postgres,tempbottle/rust-postgres,achanda/rust-postgres,ddrmanxbxfr/rust-postgres,sfackler/rust-postgres,jmesmon/rust-postgres,achanda/rust-postgres,jmesmon/rust-postgres,tempbottle/rust-postgres,ddrmanxbxfr/rust-postgres