tcp: prevent timer handler being called frequently

tcp: prevent timer handler being called frequently

In the current implement, tcp would start or up an one tick
retransmit timer for that connection if vlib_buffer_alloc()
return 0. Now the tick is 0.1ms, this means that if VPP is
in a buffer shortage state, there would be a large number of
burst timer expirations.

This commit limits the minimum interval of the retransmission
timer to 100ms.

Type: fix

Signed-off-by: liuyacan <liuyacan@corp.netease.com>
Change-Id: Ia11d693fe46119c5dc16b24ca93c30c31109057a