11709 tile access

Read access to pyramids was protected be a "rw" lock on `RandomAccessFile`.
(Acquiring just a "r" lock is not possible.) This properly failed when the background
thread was generating the pyramid, but also periodically failed when:
 * several threads tried to load the pyramid at the same time (as clients do)
 * the server was running somewhat slow.

This PR disables the read locking and simply checks for the existence of the file
as sufficient safety. It remains to be shown that this is "safe enough".

/cc @chris-allan @will-moore @pwalczysko