Cargo.toml: Update dependencies

Support postgres with SSL requirements

This stupidly just strips any ?sslmode= arguments from the postgres URI
and enables SSL without verification regardless of which mode the user
specifies.

Add information from #2 to README about being race-safe

Modernize the code

From Jörg Sommer's PR:
https://github.com/matrix-org/rust-synapse-compress-state/pull/10

Some compressions yield only a few hundred lines saving which isn't worth
the database update and the time needed to validate the compression. With
the new commandline argument `-m` the output would be suppressed, if not
at least the given number of database records would be saved.

To unify the spelling of parameters, i.e. all upper-casse, add the
`value_name()` for max_state_group.

`cargo clippy` suggested to use a character instead of a string with a
single character.

See: <https://rust-lang.github.io/rust-clippy/master/index.html#single_char_pattern>

`cargo clippy` suggested to use `is_empty()` instead of `len() > 0`,
because this doesn't require to calculate the whole length of the sequence.

See: <https://rust-lang.github.io/rust-clippy/master/index.html#len_zero>

Delete Cargo.lock and re-add it after a `cargo update`. Since we just
did the latter, the versions referenced do not change.

Signed-off-by: Olivier Wilkinson (reivilibre) <olivier@librepush.net>

Fix infinite loop when looking for missing state groups

The code which handles state groups which appear in state_group_edges but not
state_groups_state would get confused if such state_groups did not themselves
have a prev_state_group.