Clean up linting warnings: replace usage of explicit lifetimes with elision.

Commands:

    cargo clippy --tests --no-deps --workspace

Warnings:

    warning: the following explicit lifetimes could be elided: 'a
       --> database/src/fs.rs:348:6
        |
    348 | impl<'a> FilesystemTransaction<'a> {
        |      ^^                        ^^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
        = note: `#[warn(clippy::needless_lifetimes)]` on by default
    help: elide the lifetimes
        |
    348 - impl<'a> FilesystemTransaction<'a> {
    348 + impl FilesystemTransaction<'_> {
        |

    warning: the following explicit lifetimes could be elided: 'a
       --> database/src/sqlite.rs:141:6
        |
    141 | impl<'a> DatabaseTransaction<'a> for SqliteTransaction {
        |      ^^                      ^^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
    help: elide the lifetimes
        |
    141 - impl<'a> DatabaseTransaction<'a> for SqliteTransaction {
    141 + impl DatabaseTransaction<'_> for SqliteTransaction {
        |

    warning: the following explicit lifetimes could be elided: 'a
      --> src/dump.rs:40:6
       |
    40 | impl<'a> std::fmt::Display for SessionKeyDisplay<'a> {
       |      ^^                                          ^^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
       = note: `#[warn(clippy::needless_lifetimes)]` on by default
    help: elide the lifetimes
       |
    40 - impl<'a> std::fmt::Display for SessionKeyDisplay<'a> {
    40 + impl std::fmt::Display for SessionKeyDisplay<'_> {
       |

database/src/fs.rs

@@ -345,7 +345,7 @@ pub struct FilesystemTransaction<'a> {
     _flock: FlockMutexGuard,
 }
 
-impl<'a> FilesystemTransaction<'a> {
+impl FilesystemTransaction<'_> {
     fn link_email_vks(&self, email: &Email, fpr: &Fingerprint) -> Result<()> {
         let path = self.db.fingerprint_to_path_published(fpr);
         let link = self.db.link_by_email(email);

database/src/sqlite.rs

@@ -138,7 +138,7 @@ fn query_simple<T: rusqlite::types::FromSql>(
         .expect("query exection must not fail")
 }
 
-impl<'a> DatabaseTransaction<'a> for SqliteTransaction {
+impl DatabaseTransaction<'_> for SqliteTransaction {
     type TempCert = Vec<u8>;
 
     fn commit(self) -> Result<()> {

src/dump.rs

@@ -37,7 +37,7 @@ pub struct SessionKeyDisplay<'a> {
 }
 
 /// Print the session key without prefix in hexadecimal representation.
-impl<'a> std::fmt::Display for SessionKeyDisplay<'a> {
+impl std::fmt::Display for SessionKeyDisplay<'_> {
     fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
         let sk = self.csk;
         write!(f, "{}", hex::encode(&sk.session_key))