Fix dialyzer errors

[joaohf]

==> Checking 240 files in _build/default/rebar3_24.3.4.16_plt...
===> Doing success typing analysis...
===> Resolving project warning files...
===> Analyzing 8 files with _build/default/rebar3_24.3.4.16_plt...

src/canister.erl
Line 63 Column 1: Function start/0 has no local return
Line 237 Column 5: Call to missing or unexported function
canister_sync:send_delete/2
Line 266 Column 1: Function update_delete_time/1 has no local return
Line 269 Column 1: Function update_delete_time/2 has no local return
Line 272 Column 21: Record construction
#canister_times{last_access::'undefined',last_update::'undefined'}
violates the declared type of field
last_access::{non_neg_integer(),non_neg_integer(),non_neg_integer()} and
last_update::{non_neg_integer(),non_neg_integer(),non_neg_integer()}
Line 333 Column 13: The pattern {'resync', Node} can never match the
type 'ok'
Line 346 Column 9: The pattern {'ok', , Node} can never match the type
'ok'
Line 375 Column 17: The pattern {Node, NewLastAccess} can never match
the type {'ok',,}

src/canister_app.erl
Line 12 Column 1: Function start/2 has no local return

src/canister_config.erl
Line 20 Column 1: Function summarize/0 has no local return
Line 38 Column 15: The call io:format(Msg::[[[any()] | char()],...])
breaks the contract (Format) -> 'ok' when Format :: format()

src/canister_sup.erl
Line 16 Column 1: Function start_link/0 has no local return

src/canister_sync.erl
Line 202 Column 1: The pattern [] can never match the type
nonempty_maybe_improper_list()
===> Warnings written to _build/default/24.3.4.16.dialyzer_warnings
===> Warnings occurred running dialyzer: 13