Complete comment identity with the associated serverId in comments

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The remapping of account-ids performed with I4b0fe6689 has correctly
adapted the comment's author account-ids to the local associated
accounts, however, it failed to update the comment serverId, leaving
the rest of Gerrit with an inconsistent identity.

Even though the identity was not causing apparent damage, mostly
because the serverId was ignored mainly by Gerrit; it left the cached
entities with the incorrect data and, in some cases, persisted back
to NoteDb (see Issue 318745940).

Rectify the situation by updating the serverId when accounts are
remapped.

Release-Notes: Fix serverId in identity parsing for imported human comments
Forward-Compatible: checked
Bug: Issue 318745940
Change-Id: I5904dc46120858beb54e20ac339f10a02d785d3a