Upgrade gwtorm to 1.7.3

Fixes regressions introduced in 1.7.1 and 1.7.2

Change-Id: If288b3a1e17715e9e511aed1869d1ad32b80444c
Update version to 2.8.6.1

Change-Id: I8669d1b25ea3b2b1e3449f449b88427193cbe822
Remove reference to MyISAM from database setup documentation

From 2.8.6 only transactional databases are supported.

Change-Id: Ie0d94c40a371dd018b897113eca8b745a1000d2e
Release notes for Gerrit 2.8.6.1

Change-Id: I625ac1ad8b84e81902d4c936c214ff242098de2f