[ref] gva: move db handlers to writer crate

Cargo.lock

rust-bins/duniter-dbex/Cargo.toml

rust-bins/duniter-dbex/src/migrate.rs

rust-libs/modules/gva/Cargo.toml

rust-libs/modules/gva/db-writer/Cargo.toml

rust-libs/modules/gva/db-writer/src/lib.rs

rust-libs/modules/gva/src/lib.rs