sqliterepo/cache: prevents threads starvation

Get rid of the old model with a fixed-number of GCond used to bucketize
all the DB accesses.

Now one couple of GCond (normal, prio) is associated to each cache item,
and this is better since the lifetime of each item is the whole
execution.

In addition, the notion of maximum bases used has been reviewed to allow
slight changes on the effective maximum. Now we propose one "hard"
maximum used to allocate structures at init time, and one "soft" maximum
that can be freely changed between (0,hard].

BUILD.md

@@ -163,6 +163,10 @@ Used by `gcc`
 
 | Macro | Default | Description |
 | ----- | ------- | ----------- |
+| SQLX_HEAVYLOAD_ALERT | TRUE | Should the server fail a request for the purpose of a heavy duty on a base |
+| SQLX_HEAVYLOAD_FAIL | FALSE | Should the server alert (if not failing) for a heavy duty on a base |
+| SQLX_MAX_WAITING | 16 | How many waiters on a cache slot necessary to consider the duty is heavy |
+| SQLX_MAX_BASES | 8192 | Default max number of bases, if not overriden at runtime. |
 | SQLX_SYNC_DEFAULT_ZK_TIMEOUT | 8765 |  |
 | SQLX_DIR_SCHEMAS | NULL | Default directory used to gather applicative schema of SQLX bases. NULL by default, meaning that no directory is set, so that there is no attempt to load a schema. |
 | SQLX_ADMIN_PREFIX_SYS  | "sys." | Prefix used for keys used in admin table of sqlite bases |

core/internals.h

@@ -163,6 +163,7 @@ enum {
 	CODE_NOT_IMPLEMENTED = 501,
 	CODE_PROXY_ERROR = 502, // unknown error when contacting an other service
 	CODE_UNAVAILABLE = 503,
+	CODE_EXCESSIVE_LOAD = 504,
 
 	CODE_PLATFORM_ERROR = 600,
 };