serv/db: Fix looping upon database locked issues

[Richard Purdie <richard.purdie@linuxfoundation.org>]

If the database is locked we will get an immediate error indicating so,
there is no retry timeout. The looping code is therefore useless, the loop
count is near instantly exceeded.

Using a time based retry means we can wait a sensible time, then gracefully
exit.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
---
diff --git a/bitbake/lib/prserv/db.py b/bitbake/lib/prserv/db.py
index 7bc1980..b7190ba 100644
--- a/bitbake/lib/prserv/db.py
+++ b/bitbake/lib/prserv/db.py
@@ -2,6 +2,7 @@ import logging
 import os.path
 import errno
 import prserv
+import time
 
 try:
     import sqlite3
@@ -32,13 +33,13 @@ class PRTable(object):
 
     def _execute(self, *query):
         """Execute a query, waiting to acquire a lock if necessary"""
-        count = 0
+        start = time.time()
+        end = start + 20
         while True:
             try:
                 return self.conn.execute(*query)
             except sqlite3.OperationalError as exc:
-                if 'is locked' in str(exc) and count < 500:
-                    count = count + 1
+                if 'is locked' in str(exc) and end > time.time():
                     continue
                 raise exc