SQL api is used to determine the list of source tables affected
by a query. Before this commit, the X-Cache-Channel header set
on sql api error was an arbitrary 'table' string, now the header
is omitted, the error logged and Cache-Control and Pragma headers
are sent as an attempt to request no caching.
The code includes test for this mechanism.
The number gives the maximum number of contemporary connections and
is dimensioned on the limit of open file descriptors found to be
needed on a per-connection basis.
It took a lot of time to produce a testcase for this as the test
config was setting srid to 4326 but not changing geom column name
thus all tiles fetched by tests returned blank (ouch!)
