This commit specifies two bbolt options when opening the underlying
channel and watchtower databases so that there is reduced heap
pressure in case the bbolt database has a lot of free pages in the
B+ tree.
This commit renames the variables dbName to towerDBName and dbVersions
to towerDBVersions, to distinguish between the upcoming clientDBName
clientDBVersions. We also move resusable portions of the database
initialization and default endianness to its own file so that it can be
shared between both tower and client databases.