refs/reftable: introduce "reftable.lockTimeout"

When multiple concurrent processes try to update references in a
repository they may try to lock the same lockfiles. This can happen even
when the updates are non-conflicting and can both be applied, so it
doesn't always make sense to abort the transaction immediately. Both the
"loose" and "packed" backends thus have a grace period that they wait
for the lock to be released that can be controlled via the config values
"core.filesRefLockTimeout" and "core.packedRefsTimeout", respectively.

The reftable backend doesn't have such a setting yet and instead fails
immediately when it sees such a lock. But the exact same concepts apply
here as they do apply to the other backends.

Introduce a new "reftable.lockTimeout" config that controls how long we
may wait for a "tables.list" lock to be released. The default value of
this config is 100ms, which is the same default as we have it for the
"loose" backend.

Note that even though we also lock individual tables, this config really
only applies to the "tables.list" file. This is because individual
tables are only ever locked when we already hold the "tables.list" lock
during compaction. When we observe such a lock we in fact do not want to
compact the table at all because it is already in the process of being
compacted by a concurrent process. So applying the same timeout here
would not make any sense and only delay progress.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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