L3 agent: paginate sync routers task

L3 agent: paginate sync routers task

In case there are thousands of routers attached to thousands of
networks, sync_routers request might take a long time and lead to timeout
on agent side, so agent initiate another resync. This may lead to an endless
loop causing server overload and agent not being able to sync state.

This patch makes l3 agent first check how many routers are assigned to
it and then start to fetch routers by chunks.
Initial chunk size is set to 256 but may be decreased dynamically in case
timeouts happen while waiting response from server.

This approach allows to reduce the load on server side and to speed up
resync on agent side by starting processing right after receiving
the first chunk.

Closes-Bug: #1516260
Change-Id: Id675910c2a0b862bfb9e6f4fdaf3cd9fe337e52f